Laugh in the good way: anything by Tom Lehrer, Crotchduster, Ninja Magic, Lawnmower Deth, Invasion of the Mutant Space Bats of Doom, Blackthrone, Spinal Tap, Metal Duck, Evig Kveld, Satan's Penguins or Kompressor. Much Beatallica, Anal Cunt, Dwarves and GWAR. A lot of Anti-Nowhere League and Stormtroopers of Death (particularly Pre-Menstrual Princess Blues, The Ballad of Jimi Hendrix and Celtic Frosted Flakes).
There's lots that makes me smile with glee, but not really laugh. Anything with King Diamond in it, for example.
As for music to laugh AT, well, I'm not sure, I think a lot of music's bad, but I normally spit rather than laugh.
Actually, thinking about it, Bullet For My Valentine made me laugh whilst crying tears of despair for modern rock music. And when I saw Atreyu live, they had an emo roady, bawling out "TESTIIING! TEEEEESTING!!!" Which was highly amusing. Actually, if you can get over how depressing it is that it actually exists, a lot of emo is pretty funny.
Also, now I'm on a roll, bad power metal bands can be most amusing, but I tend to think of their inadequacies as charming. I mean, I'd pay money to see Winterlong in concert.
Yeah, I know I'm an idiot.