http://gnufoo.org/ucontrol/ - GPL
allows you to remap capslock to control. also gives you the ability to do Fn+trackpad as a virtual scroll wheel function.
free 600MB+ by deleting support for languages and input methods that you know you will never use.
http://homepage.mac.com/mscott/starfishx/starfishx.html - GPL
generates interesting background patterns.
http://www.unsanity.com/haxies/shadowkiller/ - freeware
removes the shadows around windows, can speed up the interface.
Text Extras - http://www.lorax.com/FreeStuff/TextExtras.html - but it supposes to give all Cocoa-based apps emacs-like keybindings in input dialogs, and access to several internal features.
opensource screen savers -
http://s.sudre.free.fr/Software/WBSoftware.html - open source
a bunch of screensavers. the SaveHollywood screen saver is interesting because it can display a flash animation or quicktime movie. which can be useful for product demonstrations or a mac-based kiosk.
http://www.titanium.free.fr/english.html - freeware
lets you control many internal features (mostly gui features) of OSX.
gives applications that support it a configurable way to provide alerts (instead of having bouncing icons in the dock)
FullScreen Player -
lets you watch quicktime movies full screen without buying the full version of quicktime ($12 shareware). you could probably spend a couple hours looking at the source to SaveHollywood screen saver and write your own thing that does the same.
http://www.bresink.com/osx/TemperatureMonitor.html - (shareware?)
watch the temp of your Mac. g3/g4 can only watch the harddrive temp. g5 has an on-die cpu temp and case temp as well.
Fink - http://fink.sourceforge.net/ debian-like package installer for various opensource apps.
Darwin ports -
freebsd-like package installer for various opensource apps.
MPlayerOSX - http://mplayerosx.sourceforge.net/ the #1 video player on Linux comes to OSX. With the interesting ability to play movies in place of the desktop wallpaper. Note that burried within the .app directory there is a normal mplayer binary that you can symlink to and use in the traditional Unix way.
divx codec for quicktime - http://www.divx.com/divx/mac/
xvid codec for quicktime -
http://people.ee.ethz.ch/~naegelic/download/ - open source
a video codec for mpeg4
convert between video/audio formats
Mac OS X speed FAQ - http://www.macmaps.com/Macosxspeed.html
allows much more customized control over the samba configuration in OSX. share directories other than Public/ folders
less/more pager in a gui
nmap in cocoa. a bit more powerful than the "Port Scan" tab in NetworkUtility
the best way to create ZIP files in OSX (opensource)
vnc server for OSX. this company sells an interesting gui testing product that uses vnc to test Mac, Windows, Linux, etc.
http://wasters.org/freecell/ - open source
finally the last reason to use MS Windows has been removed.
MacOS X Hints -
A hodgepodge collection of articles covering almost every issue you have ever had with your Macintosh.
http://www.wolfware.com/wClock.html - freeware
A nice simple menubar clock application that is substantially less braindead than Apple's.