IIRC
IMO ....in my opinion
IMHO .....in my humble opinion
IDB ....in da butt
STFU ....shut the fuk up
IB ...in before
IMM ....i miss my
TTYL ....talk to you later
GTG .....got to go
GG ...ga ga
WTF ...what the fuk
L337 ...elite
OMG .....oh my god

-edit. i thought you wanted to know what they meant h: what is iirc?