Shush
Shush is a utility app for Mac OS X to quickly mute and unmute your microphone using a hotkey.
SelfControl
SelfControl is a free and open-source application for Mac OS X (10.5 or above) that lets you block your own access to distracting websites, your mail servers, or anything else on the Internet. Just set a period of time to block for, add sites to your blacklist, and click "Start." Until that timer expires, you will be unable to access those sites--even if you restart your computer or delete the application.
Random Genius: Safari 4 Hidden Preferences
Maybe useful at some point.
The MacPorts Project
"The MacPorts Project is an open-source community initiative to design an easy-to-use system for compiling, installing, and upgrading either command-line, X11 or Aqua based open-source software on the Mac OS X operating system."
funkatron.com : Assumptions and Mac browser share
Firefox has more users, perhaps, than is commonly assumed. With Firefox 3 on the near horizon, this may shoot up even more.
Paparazzi!
Free Mac software for taking webpage screenshots. Can capture the entire length of a page (beyond the "fold").
HandBrake
"HandBrake is an open-source, GPL-licensed, multiplatform, multithreaded DVD to MPEG-4 converter, available for MacOS X, Linux and Windows."
Many Tricks
Some very useful utilities. Service Scrubber helps you manage OS X services, Witch turns command+tab into something that can access the windows of individual applications, not just the applications themselves.