FREE SOFTWARE ...because the software police will eventually carry mace
Compatibility: = OSX = Windows =Linux =OS9
Firefox - Version 3 is looking better than ever. The web browser Firefox has a great support community, and a great deal of useful extensions (like Adblock, and FireBug). Aside from the now standard tabbed-browsing, this new version has tagging for bookmarks and a slick auto fill URL bar. Camino is an OSX only version of Firefox which has more of an aqua look and definitely worth a look.
Thunderbird - This open source email client is maintained by the same community that develops Firefox (Mozilla).  It is a very clean and easy to use email program.  Basically all the best parts of Outlook, without the clutter.
OpenOffice  - The open source alternative to Microsoft's Office suite of programs.  Open Office can read/write the common Microsoft formats (Including Office 2008 formats), and has the useful ability to save slide shows as flash movies. Unfortunately it is not an OSX native program and must be run inside the the X11 window system. See NeoOffice below.
NeoOffice - What could be better than the free feature-rich OpenOffice program suite? How about an OSX native version (no need for X11)! In addition to all the functionality in Open Office, NeoOffice integrates dozens of native Mac features.
Kompozer - A powerful yet easy to use WYSIWYG web page creator suited for students and teachers.  Great for districts who do not wish to pay for costly HTML editors.  It includes much of the options that you find with expensive programs like Dreamweaver such as: CSS editing, FTP client, webdev support, and site management. Kompozer is basically the newer version of the program NVU, and fixes many of the bugs in that program.
ClamXav - Free virus scanning for OSX.  Yes it's true that viruses, for the most part, do not effect OSX users.  However, since most of the world uses Windows it is a good idea to make sure you are not emailing something harmful to your fiends and colleagues.  It also will scan for macro viruses which are a problem if running Microsoft Office on a Mac.
AVG Free Addition - Have Windows?  You need to worry about all kinds of security problems.  AVG will do a decent job of keeping your computer virus free... for free!
Please note that the free addition is only licensed for home/personal use.
Galerie - A favorite of elementary teachers who take large amounts of pictures.  Galerie is an easy way to make great looking web pages to showcase pictures.  Just point the program to either a folder of pictures or iphoto album, specify a few settings, select a template, and the program will create a nice looking web site with click-able thumbnails of the photos.  All you have to do to publish is throw the entire folder created on the district web server.  There are a large number of user created templates you can download from the publisher's web site.
GIMP - The GNU Image Manipulation Program. Or as I like to think of it as the open source equivalent of Photoshop. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. It works on many operating systems, in many languages. Unfortunately, like OpenOffice, GIMP must run inside the X11 window system if used on a Mac.
SeaShore - Still in beta, Seashore is an OSX native image editor based on the popular unix program GIMP. Unlike GIMP, SeaShore does not need to run inside X11.  The program has the look and feel of Photoshop Elements,  and shares many of photoshop's features such as: layering, alpha masks, and channels.  Files can be saved as TIFF or JPEG.  Great for middle school classes that want to do more advanced image editing.
SMARTReporter - A small program that runs in the background and periodically checks the SMART status of your computer's hard drive/s.  SMART is a technology installed in the firmware of hard drives which detects hardware failures, or impending failures.  SMARTReporter can (in most cases) warn you of hard drive failure before it happens.
TextWrangler - A free scaled down version of the popular BBedit program. A big help for projects that require a pure text editor (one that does not apply formatting), such as unix scripting, javascript, plist editing, or HTML coding.  See their site for the long list of features.
Carbon Copy Cloner and Netrestore - Both written by a Mac administrator's best friend... Mike Bombich.  I'm not sure how anyone could manage hundreds of Macs without the use of Netrestore.  Much like Norton Ghost for windows, Netrestore allows you to create disk image files and "clone" computers through a network connection.  Carbon Copy Cloner does the same but between two local disks.
PHPiCalendar - So I have my school's event calendar in iCal and want to post it to the district's web site. Well PHPiCalendar was created for just that purpose.  The program is simply a folder full of PHP pages that will read an .ics file and make it into a web site, with all the navigation built in.  PHP iCalendar is compatible with a large number of calendaring programs other than iCal.  For a full list click HERE .
Mouse Locator - Great for presentations or demonstrations on the computer. Mouse Locator will put an target graphic around your mouse cursor whenever a set hotkey is used. You can set the duration, intensity, and hotkey combination.  In addition, Mouse Locator allows you to chose which graphic is used and weather or not mouse clicks should be highlighted.
QuickSilver - Possibly my favorite utility ever created for OSX.  It is hard to describe what the program does exactly.  But I guess it could be called a application launcher.  Basically you press the specified shortcut key, and then start typing the title of what you want to launch be it a program, address book entry, bookmark, etc.  Sounds simple, but it can have a big impact on the way you navigate OSX.
Audacity - A very nice interface for editing sound files, or recording voice.  It is a great choice for teachers interested in doing a podcasting project.
Mactracker - A helpful program for people in charge of maintaining or repairing Apple products.  It is basically a database of all the specifications for every Apple product ever made. Constantly updated with new products by the developer.
Smell-O-Mints - The periodic table of elements never looked so good. With an aqua and brushed metal interface, this periodic table will display basic characteristics of each element when you click on them.
Onyx - There are many system utilities people have written to access the hidden command-line options in OSX.  Onyx is my favorite of the bunch.  It allows you to do such things as show hidden files/folders, deactivate dashboard, alter the login window, increase the speed of Safari, clean every cache, run maintenance scripts, etc.
Mindcad TIler - A small application used to break up and print PDFs on multiple 8.5 x 11 pages. An easy way to make those long banners that line the school hallways.
AbiWord - Yet another free alternative to Microsoft Word.  AbiWord is a full featured word processing program with intuitive controls and a clean layout, but minus some of the bells and whistles included in Word.  It can save documents in a variety of formats including doc, rtf, and the increasingly popular opendoc. In addition, it is the only free program that I have encountered which reads Word Perfect Documents.
Image Tricks - Is not so much a image editing program as it is a image manipulation program. With Image Tricks you can apply a large number of effects to pictures using the core image filters built into OS 10.4.
CyberDuck - An easy to use FTP client for OSX. Even comes with a widget interface and example applescripts showing how to automate FTP transfers.
Wink - One piece of freeware I really wish they made for Mac. Wink is a Tutorial and Presentation creation software, primarily aimed at creating tutorials on how to use software (like a tutor for MS-Word/Excel etc).  Since you can save in flash format Wink makes it extremely simple to publish your tutorials to the web.
Picasa - An excellent piece of software made by the folks at Google to organize, edit, and share your digital photos. Very much the equivalent to iphoto for those poor people not fortunate enough to have a Mac.
Disk Inventory X and WinDirStat - Both programs are similar in function.  DiskInventory X works in OSX, WinDirStat in Windows.  When opened both programs index the files and folders on your hard drive (this could take awhile), then display the sizes of said files and folders graphically.  This helps answer two important questions:
1.  "Who is taking up all the space on my file server" - Network Admin
2.  "How do I get rid of this message saying my startup disk is full" - User
MAMP - Stands for Macintosh Apache Mysql and Php.  The ultimate web programming test tube.  This is a self-contained launcher for these three programs.  If you administer an OSX or Unix based web server these are the tools of your trade.  But MAMP makes it simple to install and configure these services on an OSX client computer to use as a test server.  I say test server because the makers of MAMP recommend against using it as a production server.
VLC - An open source media player capable of playing a wide verity of video formats.  Definitely a good replacement for the sometimes clumsy Windows Media Player, which is no longer being updated for OSX.
TuxPaint - A kind of open source version of KidPix.  TuxPaint is a drawing program for children, and as such has big easy to use controls, lots of color, and lots of sound effects.  The developer's site has some great videos of the software in action. While your there remember to download the additional rubber stamps for the program.
PDF Creator - Every Mac user knows that you can create a PDF directly from any print job in OSX.  But did you know you can get the same functionality in Windows?  Enter PDF Creator.  A free tool which will act as a virtual printer, and create PDF files from any print job.
Another PDF program worth mentioning is the lightning fast Foxit Reader for Windows, which will open PDF documents in a fraction of the time it takes Adobe Reader.
SirAdmin - A GUI interface for the command-line utility cyradm.  Cyradm is what allows an network admin to directly manipulate the Cyrus IMAP server which runs the mail service on OS X Server. Cyradm is currently the only way to do many advanced email tasks such as renaming, reconstructing, and deleting mail stores (they do not get removed with a user), sharing non-admin mailboxes through the use of ACLs, etc. The program requires you are running OS 10.4 on the client, even though it can talk to OS 10.3 servers.
SyncBack - A simple yet powerful backup utility for Windows.  Can be used to backup or synchronize critical folders to backup drives, or network shares.  Contains many advanced options like email notifications, encryption, compression, filtering, and will even close active programs if you like.
KeePass - A lightweight open source password manager.  The perfect way to keep track of passwords and other sensitive notes.  KeePass is a strongly encrypted database which is searchable, portable, and is unlocked with a single master password or password disk (for the 007 types).
NHR - Network Home Redirector is a series of install packages for OSX which simplify's the task of redirecting the cache stores for things like fonts and Microsoft Office when you are logged into a network home folder. Before now this process was complex to setup, but essential in districts using Network home folders in conjunction with Microsoft office.
SpamPal - An in genius open source program which works to filter out spam email on your Windows computer.  The program works by actively monitoring traffic coming into the POP and IMAP ports, and therefore does not need to be setup for a specific email client (making the setup very simple and effective).
TrueCrypt - Creates encrypted virtual disk files for storing data which must be kept secure even in the event that your computer is stolen. The program can also completely encrypt an entire disk, such as a USB key chain.
Iceberg - A program which helps in the creation of OSX installer packages (.pkg files). Combined with Apple Remote Desktop, package files can be a powerful tool in the management of large scale Mac deployments.
MyHome - An excellent addition to any elementary school who uses locally managed accounts in OSX and has students manually mount their home folders.  Just put MyHome in the dock, and no more "I forgot how to get to my server."
JumpCut and Clipboard Buddy- Both programs are what is commonly called clipboard-buffers. This means they will quietly sit in the background on your computer and store things that you cut or copy to the clipboard. After using one for a bit you will find yourself wondering how you ever got by with only copy/pasting one bit of text at a time. JumpCut is for OSX and Clipboard Buddy is for Windows.
Gawker makes time-lapse photography/movie making extremely easy using the built-in iSight on the new macs, or any attached camera. Just input the interval between frames, and what movie output you would like. Interesting also is it's ability to attach to remote cameras attached to other macs through bonjour.
iRed Lite greatly extends the use for that tiny Apple remote that came with newer Macs. It allows the remote to control many system actions and programs including PowerPoint, NeoOffice, and QuickTime. Comes in very handy for presentations in and out of the classroom. The remote buttons can also be mapped to applescripts (the possibilities are endless).
SpiceWorks is a network monitoring and inventory type software.  It can track and give all kinds of details about nodes on your network (PCs, Switches, Routers, Printers, etc). Unfortunately it must be run from Windows. But it can report on OSX, Linux, and Unix stations just fine.
PHPLIST - An open source news letter manager which is easily integrated into an existing school/district web site. It features RSS aggregation, scheduling, mailing-list opt in, tracking, and a bunch of other useful functions.
Books - Have a large personal or classroom library? Books is an open source program for cataloging and lending your... Books. The program can use any attached camera (including isight) to scan the barcodes on your books and auto fill information about them from a variety of sources.
Check Off - is a to-do list that sits in your Mac's menu bar. Simply click the check mark and your personal to-do list will drop down. Simple and unobtrusive. And supports the iPod's notes feature.
MorphX - Use Norrkross MorphX for calculating images or movies where one image transforms into another. Simply drag 2 images to the document window, drag a few lines to indicate similar areas and Norrkross MorphX calculates images so that it looks like the first image magically is transformed into the other. You can save the morph as as still image snapshot or as a full animated QuickTime movie.
Free Ruler - The name says it all. Measure things in pixels, inches, etc with this nice looking screen ruler. Found it helpful for object placement in web development. And in the creation of object reports for powerschool.