Just when you though French couldn't become a more complicated and slightly annoying (to learn) language, let me introduce you to Verlan. Verlan is more than just slang, it's a different language on it's own. To compare it to something in english, we could call it the Pig Latin of French. Bascially, in order to speak Verlan, you must do three things, split the word in question into it's syllables, reverse them, and then condense them into the simplest version of the word possible.
An example of verlan is the word Verlan itself, it comes from the word l'envers (meaning reverse)... the deconstruction goes something like this:
l'envers... l'en vers... vers l'en... versl'en... verslen... verlen . . .
until finally we come to Verlan.
CRAZY right? I never noticed people speaking Verlan while I was in Bretagne, I heard lots of slang and shortened words...but I don't think it was Verlan. Though it's possible my brain just couldn't pick up on it. I don't think it's that common (maybe some words though) or maybe among criminals and kids trying to be cool? I don't know, but it's a whole other dimension in the french language.
About.com explains it even better than I can, with a million great examples, so if this peaks your interest visit that page here.
