TuxMath

tux4kids.alioth.debian.org
Mac OSX, Windows, Linux
TuxMath, lets kids hone their arithmetic skills while they defend penguins from incoming comets, or offers them a chance to explore the asteroid belt with only their factoring abilities to bring them through safely!
PDFsam

www.pdfsam.org
Mac OSX, Windows
A handy little open source program that splits and merges PDF documents. It can split on bookmarks, chapters, etc. You can visually rearange PDF pages. Or even create a new PDF dragging in pages from a bunch of other PDFs.
Site Sucker

www.sitesucker.us
Mac OSX
SiteSucker is an OSX application that automatically downloads Web sites from the Internet. It does this by asynchronously copying the site's Web pages, images, backgrounds, movies, and other files to your local hard drive. Just enter a URL, press return, and SiteSucker can download an entire Web site.
Startup Inspector

download.cnet.com
Windows
Startup Inspector can illuminate why your PC is running so slow. Generally, every program you install seems to think it is so important it needs to run all the time. Let Startup Inspector explain what those programs are and if they are really needed.
There are many programs out there like this for Windows. I just find this one to have the most useful and clean interface.
Xmarks

www.xmarks.com
Mac OSX, Windows, Linux
Xmarks is a free add-on for your browser that synchronizes and backs up your bookmarks across multiple computers. Works in just about every browser (Firefox, IE, Chrome, Safari). Nicer still you can access your bookmarks even when your not on your computer\s via your account on xmarks.com.
Notepad++

notepad-plus.sourceforge.net
Windows
A powerful yet lightweight source code editor for Windows. One that I used profusely back before my Mac-conversion. Notepad++ as the name implies is simply a replacement for Notepad with an immense number of features: syntax highlighting/spacing, WYSIWYG editing, auto-complete, macro recording, etc.
VitualBox

virtualbox.org
Mac OSX
A nice little OS emulation software from Sun Microsystems. Run Windows, Linux and Unix inside of OSX.
SuperDocker

www.superdocker.com
Mac OSX
Add a little extra style to your Mac by changing the dock background. The utility also has a few other system tweaks for the look of Dashboard and Safari.
EverSave

www.tool-forcesw.com
Mac OSX
EverSave is a clever tool which allows you to save all of your documents - if possible - in a specific time interval or by changing the frontmost application. This means no more data loss if applications crash.
Anxiety

www.anxietyapp.com
Mac OSX
A super-lightweight to-do list application for Mac OS X Leopard that synchronizes with iCal and Mail. Its aim is to provide a streamlined, easily accessible interface to add and check off your tasks.
Colloquy

colloquy.info
Mac OSX
Traditionally, chat clients on the Mac have been anything but glamorous. Colloquy is an advanced IRC, SILC & ICB client which aims to fill this void. By adhering to Mac OS X interface conventions, Colloquy has the look and feel of a quality Mac application.
Sumltron

tuppis.com
Mac OSX
Smultron is a free text editor for Mac OS X Leopard 10.5 which is both easy to use and powerful. It is designed to neither confuse newcomers nor disappoint advanced users. It should work perfectly for a whole variety of needs - like web programming, script editing, making a to do list and so on.
MacLoc

ourapples.com
Mac OSX
This simple application activates the Fast User Switching feature built-in to Mac OS X. After clicking on MacLoc, it will appear that you've logged out, however upon entering your username and password, all open documents and applications will be right where you left them. MacLoc is the perfect solution when you need to step out of the room for a moment.
NameChanger

mrrsoftware.com
Mac OSX
NameChanger is designed for the sole purpose of renaming a list of files.
Bonus, you get to See how the names will change as you type.
iClockr

www.kedisoft.com
Mac OSX
iClockr is focused to show you a simple way to track your time. Great for calculating billable hours of freelance work.
Skype

www.skype.com
Mac OSX, Windows, Linux
Voice over IP for the world. Chat, video chat or call anyone in the Skype network for free. Pay a monthly or per call fee to call regular phones and have your own number.
Adium

adium.im
Mac OSX
Consolidate all of your instant messaging accounts into one program with an outstanding interface. Adium can be used with AIM, MSN, Jabber, Yahoo, and more.
Flame

husk.org
Mac OSX
Want to see what is lurking around the background of your network? Just start Flame to see all the computers on the network that are advertising services. You can even delve right into the service record to see the gory details, if you like that sort of thing.
Google Earth

earth.google.com
Mac OSX, Windows, Linux
Well if you have yet to try out Google Earth you are missing out. This program is an amazing geographic vistualizer and mapping tool. Check out Google's education site for suggestions on how it can be incorporated into the classroom.
Scribus

www.scribus.net
Mac OSX, Windows, Linux
An open source professional page layout program, very popular on the Linux platform. Features a clean user interface and advanced functions such as color seperations.
Omni Dazzle

www.omnigroup.com
Mac OSX
Adds all kinds of fun functions to your mouse when doing presentations via a keyboard or mouse hot key.
ScreenToaster.com

screentoaster.com
Web-Based
There are quite a few free online screencasting sites out there. So far screentoaster.com takes the cake.
It is as simple as clicking record, run through your tutorial on your computer, and click stop. The recorded video can be exported to a number of formats to share with others and post online. What really sets ScreenToaster apart is it's ability to use practically and audio or video input while recording. Plus you may do simple edits to the video after recording right through their interface.

Deploy Studio

deploystudio.com
Mac OSX, Windows, Linux
Made by Apple engineers in France (though not offically sanctioned by Apple) Deploy Studio is an excellent replacemnet for the now end-of-life NetRestore. With Deploy Studio you can create a master image of any Mac or PC and clone that image through your network to any number of other computers. BUT WAIT, there is so much more. The program is designed so you may easily setup pre and post imaging events in a workflow. So as an example, you could setup a workflow for your school's video production lab that went like so:
  1. Create two partions on the drive one for OSX one for Windows.
  2. Blast my mac and PC master images to those new partitions.
  3. Install my video editing software.
  4. License said video editing software.
  5. Run some more post install scripts.
  6. Setup the proper network settings, and bind this computer to my directory.
  7. Etc, etc.
iConcertCal

iconcertcal.com
Mac OSX, Windows
Being a big concert goer I was so happy to stumble onto iConcertCal. It is an iTunes visualizer that will show you upcoming concerts and CD releases for the bands in your iTunes library. With nice rollover effects and direct links to buy tickets I think I can finally stop pouring over the early warning section of The Reader.
Firebug

getfirebug.com
Mac OSX, Windows, Linux
I consider this Firefox addon a MUST for anyone involved in website development. With it you can view and edit the source and CSS code for sites your building IN REAL TIME while viewing the pages in firefox. Use the built-in inspector to drill through your own code or to see what other sites are empoying. Try out the debugging console for javascript or PHP. Validate your code. Get that placement just right with the DOM editor... and so much more.