Applejack

Applejack is your friend if you can't boot up a GUI on your mac. Once installed, if you have a problem, boot into single user mode by hitting cmd-S and typing 'applejack'. You're then presented with a prompt which lets you repair disks, permissions, cashes and remove your swap file. Thankfully, I have never needed to test this in a real situation, but it's a comforting thing to have installed on your system.

Developer's site