I'd say LOL is pretty awesome, simply because it means 'fun' in Dutch. Also, having roleplayed for some time, it always humours me if I see IC or OOC used for anything else than 'In Character' or 'Out of Character'. Oh, and the short-hand version of Warhammer (Online): Age of Reckoning, WAR, is also quite humorous. It's a little bit predictable but still clever enough to be funny.
Edit: Oh, I forgot ISMFOF and WTFWJD, the latter being a song made by the first. "I Set My Friends On Fire" and "What The Fuck Would Jesus Do?".