These are the few common ones.... people usually use them in chats, but slowly in forums, and everywhere, now.. some people even use them iRL (in real life), as in writing letters and stuff... this, if I'm not mistaken, started from IRC (Internet Relay Chat).
lol - laugh out loud
rofl - rolling on floor laughing
lmao - laughing my ass off
roflmao - rolling on floor laughing my ass off