The Fink Project - - Unix software ported to Mac OS X, either as binaries, or from source.
FinkCommander - - A graphical user interface for Fink. (only for Jaguar, as of version 0.5.0.
HandBrake - - A GPL'd multi-platform, multithreaded DVD to MPEG-4 ripper/converter. HandBrake was originally available on the BeOS, but now has been ported over to MacOS X and to GNU/Linux.
LAMP - - An ambitious project to provide a POSIX environment on Mac OS.
The Mac GNU OS X Public Archive - - Packages of all forms of Open Source Software including many Unix applications ported to or built for OS X.
Mac OS X Man Pages - - Apple's collection of man pages available from the command line, but much easier to browse.
MacDevCenter - - Mac development tutorials, opinion, and discussion from the O'Reilly Network.
MacPorts - - Aims to provide an easy way to install various open-source software products on MacOS X.
Red Bird Island - - Free software for the Mac of various genres.