rofl

Roling on (the) floor laughing

lmao

Laughing my arse off

roflmao

Roling on (the) floor laughing my arse off

imo

In my opinion

fyi

For your information

wtf

What the (flamingo)

"PP"

Per Particle (you may find this in maya quite often when messing around with particle effects and expressions)

Tut

Not really an abbreviation, but short for "tutorial"

Sim

Another 3d term, short for "simulation"

OMGWTFBBQ!

This was previously Some Guy's (I think...) signature; while we can disect it to create "oh my god, what the (flamingo), barbeque!," you might nonetheless want to have them elaborate upon it...