] Though Webster publishes a slang dictionary, it could
] potentially take years for a new word or phrase to enter
] its pages. Now, with the power of the Internet, it can be
] in a dictionary in a matter of hours. This page depends
] entirely on your contributions. Click on the "Submit" tab
] to submit slang words or phrases you've heard.

It looks like all submissions are edited by hand, and it hasn't been updated since January 2003, but still, it's a fun resource.

Online Slang Dictionary