L'argot. Also see: verlan.

A small selection:

• baiser - to fuck. "Je veux te baiser"= I wanna shag you. "Baise-toi!" = fuck off. Note: the dictionary probably says this means to kiss - don't use it as such unless you want serious misunderstandings.
• bander - to have a hard-on. "Je bandes", "Je bandes pour toi" and "bandeur". Also débander, the reverse, commonly used as an insult - "tu débandes?" means 'are you chicken?'
• les bloblos (f)- big, fat, droopy boobies.
• bosser - to work (at your job).
• bourré(e) - pissed blind drunk.
• branleur, branleuse - wanker. Also branlage, (wanking) se branler (to wank)
• casser la gueule à quelqu'un - to beat the shit out of someone
• cette putain de machine - this fucking machine. Can refer to anything, but handy for geeks.
• la chiasse - the shits, diarrhoea. Also chier, to shit, and les chiottes, the bog (Yankee can)
• le clito - fairly obvious
• le con - cunt, or female genitalia. Also connard as insult or connasse.
• les couilles - balls
• le cul - arsehole.
• de merde - fucking, used as an intensifier, as in "putain de merde!"
• les doudounes - tits
• la drague - cruising (gay men) as in "faire la drague"
• déconner - to fuck someone around, or trick them. "Sans déconner?" = No shit?
• emmerdant - annoying. As in "Que tu es emmerdant!"
• enculer - to bugger. "Va t'enculer!" = bugger off.
• fils de putain = sonofabitch
• le flic - cop
• le foutre - sperm, jizz
• foutre - to fuck up. "C'est foutu" = it's fucked, done for. NB not used now to mean having sex(see baiser)
• gerber - to puke
• la gouine - lesbian, dyke
• jouir - to come, in the sexual sense
• le keuf - analogous to the London phrase for a cop "the filth"
• mec - man, as in Hey man. "Alors, mec!"
• merde - shit. merdeux = shitty.
• une meuf - a woman. Verlan slang; femme reversed.
• nique ta mère - Fuck your mother.(insult, not serious!)
• l'oignon - arse.
• la pipe - blowjob. "Faire une pipe" to give a blowjob.
• pipi - "Faire une pipi" - to have a wee.
• pisser - as above.
• pouffiasse - bitch, whore. Insult.
• putain - whore, but also used as a general interjection, as in "putain de merde"
• péter - to fart.
• ramoner - to screw, fuck
• salaud - bastard. Salope for a girl.
• le zizi - willy.

Nobody I know uses that slang. That's lame standard French, which is used by people in France. "putain", "mec", "meuf", that is *so* France. Here's some slang for Real Men (Quebec slang) (Warning: parental guidance suggested):

• criss: General swear word, put randomly into sentences. Not very offensive.
• ostie: General swear word. About as offensive as "damn".
• tabarnac: Another general swear word. Almost as offensive as "fuck". By the way, these first 3 are derived from names of things in church: blasphemy was offensive in the past.
• marde: Shit.
• foque: Exclamation expressing dismay. Taken from English "fuck", but not nearly as offensive.
• foule: A lot. Taken from English "full".
• hotte: Awesome, kickass. Taken from English "hot".
• coche: Cop.
• trou de cul: Asshole.
• cave: Idiot.
• con: Idiot.
• fiche: Idiot, sucker.
• buter: Beat up.
• fourrer: To fuck, in the sexual sense, or to screw up.
• bouffe: Food.
• crisser le camp: Move, leave.

  Example uses:
  L'ostie de criss de char de merde veut pas partir, criss de tabarnac!: The goddamn shit car won't start, fuck!
  Foque de merde! C'est les coches! Crisse ton camp, ostie de con!: Damn shit! It's the cops! Get out of here, you fucking idiot!
  Maudite merde qu'y est hotte c'te char là!: Shit, that car is awesome!
  Mon ostie de frère, y'est foule cave.: My goddamn brother, he's totally moronic.
  Y a fourré ma blonde!? J'vais le buter, le tabarnac de trou de cul! Ostie, là y'est vraiment dans marde.: He fucked my girlfriend!? I'll kick his ass, that fucking asshole! Damn, now he's really in the shit.
  L'ostie de fiche, y'a tout fait crisser le camp à terre pis tarbarnac, là ma bouffe est pus mangeable.: That damn idiot, he pushed everything on the floor and fuck, now my food is inedible.

  I'll add some more when I think of them :).

I'm specialized in Quebec slang, so I'll focus on the particularities from this part of the world.

Frech (from France) slang, was first used by criminals and bandits during the Middle-Age (for more information, see my french argot node). But now, all the cool teenagers use it.

Words ending in "-er" are infinitive verbs

asteure - now
attaboy! - wow (an exclamation)
beu - cop
broue - beer
ça suce - "that/it sucks"
chien sale - dirty dog or cop
crosser - masturbating or fooling someone
décalisser - wreaking havoc, breaking something
décrisser - leaving, moving away
gomme balloune - bubble gum
machine - car
mongol - stupid
niaiser - procrastinating or laughing at someone
pis - and (pronounced just like pi)
pitoune - sexy, cute pejorative
pogner - getting, handling
quétaine - ugly, old, not trendy
tapette - faggot
tsé - y'know
snoro - insidious, sneaky, tricky
vedger - doing nothing, lazyness
yeule - mouth

I'll add more of them later.