Monolingual is a program that does it, after I ran it, it said it would free up like 2gb's of space, but I only noticed about 600mb that actually got freed up on my hard drive... still, that's something. Just use it on it's defalut setting.
Also, be sure to throw away the Garage Band demo songs from the Garage band folder in your music folder. Those are huge. If you use Appzapper, you can trhow away all the programs you don't use, and it finds all the files and packages related to them in one swoop. I trashed Keynote, iMovie, iDvd, and some other things I don't ever use and never will.