I find urbandictionary.com invaluable for finding or deciphering slang. That, and my kids. Incidentally, I was at a q&a with Joe Cornish who wrote and directed Attack the Block and he said that he went down to a youth club local to where the film was set and talked to the kids there and recorded the conversations to get a handle on the kind of street slang they used. In the end though, he narrowed it down to only 6 or 7 key phrases that he used throughout the dialogue, otherwise the majority of the audience would get lost. Keeping it to a few key phrases meant it would be easier to grasp and easier to hold on to when watching the film.

I have a few for UK slang: http://www.cockneyrhymingslang.co.uk/ http://www.translatebritish.com/ http://www.peevish.co.uk/slang/ http://www.anglotopia.net/british-identity/humor/top-100-most-beautiful-british-slang-words-and-phrases/ http://www.effingpot.com/slang.shtml

I would think that Facebook and Twitter feeds would be a great resource for this. I would also say maybe the best one. If you must go to a dictionary-like source in order to give your character dialogue I argue you shouldn't be writing that character. If you haven't researched the world that character lives in enough to have their speech patterns and slang come naturally you are not ready to write. I know there is a writer out there who will take the examples above and think that they can write a authentic sounding Crip or Blood but when do they say these things? How do we incorporate it into a script so that the audience knows what we mean even if they are not a gang member? Is there a standard response? Are these slang terms only relevent to NY Crips and Bloods or do those in the rest of the country use them too? I believe that all slang resources should come with a warning for writers. They should be used like any other dictionary. Don't go rummaging through looking for things that sound cool or that is a short way to say something long if it is not organic to the story and character.

If you like working class British comedy from the 50's, 60's & 70's, usually London based, (i.e Carry On movies, Goons/Spike Milligan, etc...) then you'll recognize a lot of these words: http://en.wikipedia.org/wiki/Polari