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."
Sakuzaku: I Am Building Ruby, Rails, Subversion, Mongrel, and MySQL on Mac OS X -- Automatically (And So Can You!)
Like it says, a shell script to automatically install the perfect Ruby on Rails development environment on OS X. Useful!