Check out the list to discover what sconnie, meat raffle, neutral ground, polio water, and slugging mean in their respective U.S. cities.
And poke around the nicely designed site to read definitions of featured words, participate in a poll (how do you pronounce pecan?), and contribute a local term of your own. My contribution: hella, in the Oakland section.
UPDATE: I've fixed the first City Dictionary link (apologies!). And here's a just-published blog post by City Dictionary founder Thomas Carmona that explains the selection process for Local Words of the Year.