Vagrant is a Ruby project that lets you set up a Linux virtual machine very quickly, using VirtualBox. Ideally, at least. Getting it set up the first time on a MacBook can be tricky. Here’s how I got it running.
This assumes you’re running Vagrant in your home directory. Also, I already had VirtualBox installed.
Updated 5/30/2011: The previous instructions added ~/.rvm/scripts/rvm to ~/.bash_profile, which caused RVM’s settings to override anything in your ~/.profile. See “Gotchas” for more info.