I've been discussing the finer points of the definition of the slang term "square" (used in the social, not geometric sense), and have had the opportunity to view several "urban slang" online dictionaries. These can be particularly helpful in understanding up with anyone born prior to 1985 (it's almost like they're speaking an entirely different language).
Anyway, here are a few good ones.
UrbanDictionary.com is a user-generated website dedicated to keeping up with the dynamics of linguistics (i.e. "urban slang"). It's saturated with ads, but for most slang terms, will provide at least four-five reasonably-thought-out definitions by users. This obviously does not replace your Webster's dictionary, but can give you a pretty good idea of what people "think" the words mean.
TheSource4YM.com/TeenLingo is a youth-ministry oriented site, and has an online dictionary to help youth workers figure out what the kids they're working with are talking about.
Wiktionary has a slang category, and is formatted much like the Wikipedia website, which has been become familiar to most of us. It is also user-generated.
Finally, Answers.com, another user-generated site. If you use the search box at the top, and type in "Slang: Word" (and replace "word" with whatever term you're looking for), you're likely to get some good definitions.
Also see: Deciphering Internet Acronyms
Jennifer Pointer (e-mail) is in Tulsa, OK. She promotes a simple, a low-tech approach to effective online profile management, search engine optimization and social networking.