ROFLMAO -- rolling on the floor laughing etc. for if it was a really funny one
WTF... What the... I'm sure I don't have to explain that one.

TTFN -- haven't seen this around here but pretty common on the internet Ta Ta for now
BRB -- Be right back again common on the internet so one might run into it
I suppose one should mention SASE but I should think everyone knows it. (self-addressed stamped envelope in case anyone doesn't)
Can't think of anything else.