ROTFLN - Rolling on the floor laughing, nekkid
OMG - Oh my god/gonads (usually used by kids sixteen and younger - if you're older than this and you still use it frequently, you should be so embarassed to type like an 11 year old script kiddie)
LOL - Laughing out loud (used widely by immature kids age 7-11)
PWNED - "Owned" (Better spelled as "pwn3d", which will get you more respect - at least one fella I was arguing with on this forum doesn't know how to spell this right; he was definitely not l33t/1337)
KNNBCB - seldom used here, though often thought about - we can pass this over for the time being
LC - ditto
YSS - You're so stoopid
YST - You're so toopid
WTF - What the f*** (f*** = fish?)
YMSF - Your mother so fat
YSSF - Your sister so fat