A
*****
AFAIK - As Far As I Know
AFK - Away From Keyboard
ATM - At The Moment
ATT - At The/This Time

B
*****
BBFN - Bye Bye For Now(UK)
BBIAB - Be Back In A Bit
BBL - Be Back Later
BC - Because
BRB - Be Right Back
BSOD - Blue Screen Of Death(boot error)
BTW - By The Way

C
*****
CS - Client Side

D
*****
DL - Download

F
*****
FOAD - F**k Off And Die
FOV - Field Of View
FPS - Frames Per Second or First Person Shooter
FTR - For The Record

G
*****
GF - (depending on context)Good Fight or Girlfriend
GG - Good Game
GJ - Good Job
GL - Good Luck
GTG - (depending on context) Got To Go or Good To Go
GTH - Go To Hell

H
*****
HPB - High Ping Bitch/Bastard

I
*****
IMHO - In My Honest Opinion
IMO - In My Opinion

L
*****
LMAO - Laughing My Ass Off
LOL - Lots Of Laughs, or Laughs Out Loud
LPB - Low Ping Bitch/Bastard

N
*****
NM - Never Mind
NOYB - None Of Your Business
NP - No Problem
NS - Nice Shot

O
*****
OFN - Old Fu*king News
OMG - Oh My God
OMFG - Oh My F**king God
OS - Operating System

P
*****
PL - Packet Loss
POS - Piece Of Sh*t
POV - Point Of View
PW - Password

R
*****
ROFL - Rolling On Floor Laughing
ROFLMAO - Rolling On Floor Laughing My Ass Off
RPG - Role Playing Game
RTFM - Read The F**king Manual
RTS - Real Time Strategy (game)
RW - Roger Wilco (voice com program)

S
*****
SAD - Super Admin
SC - ShoutCast (internet match broadcasts)
SS - (depending on context)Screen Shot or Server Side
STFD - Sit The F**k Down
STFU - Shut The F**k Up

T
*****
TK - Team Kill
TMI - Too Much Information
TS - TeamSound or TeamSpeak (voice com programs)
TTYL - Talk To You Later
TWIT - That's What I Thought
TY - Thank You

W
*****
WB - Welcome Back
WTF - What The F**k
WTMI - Way Too Much Info
Here are some of the more common abbreviations you may see on the web in chats or on message boards. I had to figure out almost all of these on my own, so this list may be somewhat incomplete or in some cases  even inaccurate. If you have some that I missed, please let me know in my guestbook and I'll add them when I get the chance.