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.
LastHistory: Visualizing Last.fm Listening Histories and Personal Streams
‘LastHistory allows you to analyze music listening histories from Last.fm through an interactive visualization and to explore your own past by combining the music you listened to with your own photos and calendar entries.’
"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."
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.
"The goal of this project is to create an easy to use GUI program that enables Mac OS X user with iPods to install linux on HFS+ formatted iPods while still allowing users to keep and use the Apple Supplied OS."