Locomotive: Rails for OS X
This is nifty – Locomotive, from Ryan Raaum, a complete Ruby on Rails environment in a self-contained 30MB bundle. And when I say complete, I mean complete: Locomotive contains not only Rails itself, but the Ruby interpreter, RubyGems, the LightTPD webserver with FastCGI, the SQLite database engine, bindings for MySQL and PostgreSQL (though not the server binaries, wisely), and all the other bits and pieces needed for turnkey Rails. There’s also an expanded version of the package with even more goodies. If you have an existing Rails installation, Locomotive will run politely alongside it without messing anything up.