LMAO = laughing my arse off (sometimes combined with ROFL to make ROFLMAO)
WTF? = what the f***?
IIRC = if I remember correctly
IM(H)O = in my (humble) opinion
FWIW = for what it's worth
YMMV = your milage may vary (i.e. your experience may be different)
GF/BF = girlfriend/boyfriend

That's all the others I can think of off the top of my head...