Freeware... because the software police will eventually carry mace.

Skitch


evernote.com/skitch/
Mac OSX, Windows
Great way to take screenshots and annotate them quickly. I tend to send screenshots with any how-to email to make my point crystal clear. Skitch is now developed by the people behind Evernote. So the two programs are tightly integrated.

Caffeine


lightheadsw.com
Mac OSX
Love this program for presenting or classroom lecture. Caffeine is a tiny program that puts an icon in the right side of your menu bar. Click it to prevent your Mac from automatically going to sleep, dimming the screen or starting screen savers. Click it again to go back.

CamTwist


camtwist.com
Mac OSX

CamTwist is a software package that lets you add special effects to your video chats. It's also possible to stream your desktop and still images. With CamTwist, you can also use multiple video chat programs at the same time.

Pencil


www.pencil-animation.org
Mac OSX, Windows, Linux
Create traditional hand-drawn 2D animation (cartoon) using both bitmap and vector graphics. I love it's implementation of "onion skin" layering. Makes for cool Dr. Katz type animation.

Spybot - Search & Destroy


www.safer-networking.org
Windows
Spybot - Search & Destroy detects and removes spyware. One of the oldest and most dependable peices of software for your security toolbox.

LaTeXiT


pierre.chachatelier.fr
Mac OSX
Typeset LaTeX equations quickly and easily. Then drag/drop them into presentations or any other document.

DrawBerry


raphaelbost.free.fr/DrawBerry.html
Mac OSX
With DrawBerry, you have all the possibilities the vectorial drawing offers you in a free, easy to use and powerful application. If you don't need applications such Illustrator or Inkscape and you want to create some simple (or less simple) illustrations, DrawBerry is for you.

The Unarchiver


wakaba.c3.cx
Mac OSX, Windows, Linux

The Unarchiver is a much more capable replacement for "Archive Utility.app", the built-in archive unpacker program on Mac OS X. The Unarchiver is designed to handle many more formats than Archive Utility, and to better fit in with the design of the Finder.
UPDATE: There is also a Windows and Linux version

TrashMe


www.jibapps.com
Mac OSX
TrashMe will help you in the uninstall procedure: drop an application on TrashMe and it will find related files for you, so as to delete everything.

Anti-Spam SMTP Proxy (ASSP)


assp.sourceforge.net
Mac OSX, Windows, Linux
The Anti-Spam SMTP Proxy (ASSP) Server project aims to create an open source platform-independent SMTP Proxy server which implements auto-whitelists, Bayesian, Greylisting and multiple filter methods. An excellent choice for fighting spam at the enterprise level.

Lazarus


lazarus.interclue.com
Mac OSX, Windows, Linux

Using Lazarus, you can recover lost forms with a single click. Lazarus automagically encrypts and saves every form as you type. If the submission fails or if you forget to send it or your computer or browser crashes, then you can return to the page and recover the form data you originally entered.
Lazarus is a plugin for Firefox. It has saved me a ton of time and headaches.

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.

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.

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.

VitualBox


virtualbox.org
Mac OSX

A nice little OS emulation software from Oracle. Run Windows, Linux and Unix inside of OSX.

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.

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.

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.
OS 10.7 (Lion) has this ability built-in!

Colloquy


colloquy.info
Mac OSX

Traditionally, chat clients on the Mac have been anything but glamorous. Colloquy is an advanced IRC 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.

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.

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.

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.

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