Sauerbraten

sauerbraten.org
Mac OSX, Windows, Linux
Sauerbraten is a free multiplayer/singleplayer first person shooter, built as a major redesign of the Cube FPS.
AbiWord

abisource.com
Mac OSX, Windows, Linux
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
Cronnix

Cronnix
Mac OSX
A GUI frontend for creating cron jobs.
Mouse Locator

www.2point5fish.com
Mac OSX
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

blacktree.com
Mac OSX
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.
Cyberduck

cyberduck.ch
Mac OSX
An easy to use FTP client for OSX. Even comes with a widget interface and example applescripts showing how to automate FTP transfers.
Klondike Forever

solitaireforever.com/klondike/
Mac OSX, Windows
Fast-pased 3D Solitare.
SpamPal

www.spampal.org
Windows
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).
Iceberg

s.sudre.free.fr
Mac OSX
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.
Perian

www.perian.org
Mac OSX
Help Quicktime be all that it can be with Perian. This utility will allow Quicktime to play a wide range of video formats it otherwise would not support, like the popular DivX.
Thunderbird

www.mozillamessaging.com
Mac OSX, Windows, Linux
This open source email client is maintained by the same community that develops Firefox (Mozzilla).  It is a very clean and easy to use email program.  Basically all the best parts of Outlook, without the clutter.
Integrity and Xenu's Link Sleuth


Mac OSX, Windows
If you run a website, it is always a good idea to check peridically for broken links. These two programs are the best automated link checkers I have found. Integrity for Macs, and Xenu's for PCs.
NmapFE

faktory.org/m/software/nmap/
Mac OSX
A popular port scanning and network security utility; NmapFE for OS X is a no hassles, GUI version for us Mac users.
SpiceWorks

www.spiceworks.com
Windows
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.
Angry IP Scanner

www.angryziber.com
Mac OSX, Windows, Linux
Angry IP scanner is a very fast IP address and port scanner. It can scan IP addresses in any range as well as any their ports. Useful in analyzing firewall rules and security concerns. It is cross-platform and lightweight.
PDFCreator

pdfcreator
Windows
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.
iStache!

www.notdustin.com/istache/
Mac OSX
This programs sole purpose is to help putting staches on your photos. Nice!
OpenOffice

openoffice.org
Mac OSX, Windows, Linux
The open source alternative to Microsoft's Office suite of programs.  I have been using the software for a couple years, and have been amazed how quickly it has matured. Open Office can read/write the common Microsoft formats (.doc, .ppt, .xls), 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.  I recommend NeoOffice for the OSX folks.
Njam

njam.sourceforge.net
Mac OSX, Windows, Linux
Njam is a full-featured cross-platform pac-man-like game
MyHome

software.relevantwalk.com
Mac OSX
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."
SMARTReporter

corecode.at/smartreporter/
Mac OSX
Monitor your OS X (or OS X Server) hard drive/s. SMARTReporter runs in the background and periodically checks the SMART status of your hard drives.  SMART is a technology installed in the firmware of hard drives which detects hardware failures, or impending failures.  SMARTReporter can be set to popup alerts or send an email if there is a problem.
Wink

www.debugmode.com
Windows, Linux
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.
Disk Inventory X and WinDirStat


Mac OSX, Windows
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
Img2icns

www.img2icnsapp.com
Mac OSX
 Img2icns is a converter tool for your images. Enjoy creating thousand of icons from your pictures.
Carbon Copy Cloner and Netrestore

bombich.com
Mac OSX
Both written by a Mac administrator's best friend... Mike Bombich.  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.