Clean your Mac
But Mr. Clean is so much cooler because it's free! (Thanks to Lifehacker on the tip)
One thing I did today to clear nearly 10 gigs off my machine: I deleted 2 entire "Previous System" folders. If Apple has ever had you do the OS shuffle, setting up a new account, and leaving your old one as an archive, consider going back and deleting your old system. Were there crazy fonts in there I should have saved? Maybe. All I know is that my hard drive is a lot leaner now... I'll probably live to regret that one.