WhatDoesThatMean is "a free lexicon of English idioms, words, and catch phrases from around the world. [It] is also a 'user built' site - in addition to searching for a definition, you can add your own." The site is pretty fun and has a wide variety of slang phrases from many countries and dialects. However, what really got me hooked was yesterday's featured entry, "Santorum." ;-) (some NSFW language)

Update: It looks like WhatDoesThatMean has removed the "Santorum" entry, so you'll have to refer to the Urban Dictionary link, which is better anyway. ;-)