One of my pet peeves is the idea that there is a right (and hence a wrong) way to speak your native language. This has always struck me as snobby and, well, unscientific. There are few things I care about less than when you're supposed to use "who" or "whom". The great thing about humans is that we make stuff up: Shakespeare did it, blog commenters do it (although some worry we are running out of new material). Language, as spoken by actual humans, seems to be in a state of constant and rapid evolution. Why try to constrain all that with arbitrary rules?
A paen to this diversity is the Urban Dictionary, which attempts to collect and define slang from a myriad of English sub-cultures. Much like Wikipedia, it is largely user-driven, so, of course there's a fair bit of humorous fakes thrown into the mix (like this, for example). And of course, you've got your usual crop of slang from hip-hop and internet subcultures, like grillz and 1337. It's also disappointing, but not super-surprising, that a lot (most?) of the definitions are not-so-clever ways of saying misogynistic and homophobic things about sex. Sigh - so it goes.
After a bit of browsing, I was particularly gratified to find accurate definitions for slang words that were really common when I was a kid, but that I haven't heard since. For example, on the playground we used to say "moded" or "face!" when someone did something stupid and you wanted to make fun of them. One definition for "moded" described it as 80's San Fran hip-hop slang, which sounds about right. I pretty much haven't heard anyone actually use those words in a sentence in twenty-odd years, although they still get a laugh out of my sister.
Steven Pinker wrote an excellent book on language acquisition several years ago called The Language Instinct that is well-worth the read. He smashes the notion that "proper" language use is indicative of anything besides cultural mores, and highlights the beauty and power of often discredited linguistic traditions: African-American Vernacular English (AAVE), teen-speak, creole and pidgin languages and the like. Of course, the book is mostly about the controversial Chomskyan view that all languages share a universal grammar which arises out of our brain (and genetic) structures (which is interesting, but maybe not the whole story, according to many linguists). Although he's become something of a right-wing, ev-psych tool in recent years, TLI is still an eye-opening, egalitarian celebration of spoken language in all its diversity.