IMglish

some popular ones for reference culled over the net...
although most of them I don't use also as we also have lots of similar-looking acronyms in other industries and at work...

AAK                             Asleep at the keyboard
AAR8                           At any rate
ADAD                           Another day, another dollar
AFAIC                         As far as I’m concerned
AFAIK                           As far as I know.
AFK                             Away from computer keyboard
AJSIL                           Age/Sex/Location
ASAP                           As soon as possible
ATM                             At the moment
ATW                            Around the web
AWGTHTGTTA           Are we going to have to go through this again
AYT                             Are you there

B                                  Back
B4N                             Bye for now
BBL                              Be back later
BBS                             Be back soon
BC                               Because
BCNU                         Be seein’ you
BTDT                           Been there, done that.
BFF                             Best friend forever
BFN                             Bye for now.
BFN                             Boyfriend
BFN/IB4N                    Bye for now
BG                               Big grin
BIL                               Boss is listening
BITD                            Back in the day
BMG                            Be my guest
BOTOH                        But on the other hand
BRB                             Be right back
BTDT                           Been there, done that
BTDTGTTSAWIO        Been there done that, got the t-shirt and wore it out
BTW                            By the way
BYKT                           But you knew that

CD9                             Code 9: Parents nearby
CID                              Consider it done
COZ                             Because
CSL                             Can’t stop laughing
CU                               See you
CUL or CUL8R             See you later

DETI                            Don't even think it.
DQMOT                       Don't quote me on this.

EBCAK                         Error between chair and keyboard.
EBKAC                         Error between keyboard and chair.
EG                               Evil Grin
EOM                            End of message
EWIW                         For what it’s worth
EZ                                Easy

F2F                              Face to face.
F2T                              Free to talk
FAQ                             Frequently asked questions
FOAF                          Friend of a friend.
FWIW                          For what it's worth.
FYI                              For your information.

G2G                             Got to go
GAL                             Get a life
GD&R                          Grinning, ducking and running.
GE                               Girlfriend
GGN                            Gotta go now
GJ                                Good job
GL                               Good luck
GMTA                          Great minds think alike.
GOL                             Giggle out loud
GR8                             Great
GRT                             Great
GTG                            Got to go
GW                               Good work

H8                                Hate
HAK                             Hugs and kisses
HAND                         Have a nice day
HB                               Hurry back.
HHOK                          Ha, ha, only kidding.

IAC                              In any case
IANAL                          I am not a laywer, but...
IAE                               In any event
IAW                              In accordance with.
IC                                 I see
IDC                              I don’t care
IDK                              I don’t know
ILY                               I love you
IM                                Instant Message
IMCO                           In my considered opinion.
IMHO                           In my humble opinion.
IMNERHO                    In my never even remotely humble opinion
IMNSHO                      In my not so humble opinion
IMO                             In my opinion.
IMPOV                         In my point of view
IOW                              In other words.
IRL                               In real life

JIC                               Just in case
JK                                Just kidding
JTLYK                         Just to let you know
JOOTT                        Just one of those things.

K                                  Okay
KK                                Okay, all right.
KIS                               Keep it simple
KIT                               Keep in touch
KPC                             Keeping parents clueless.

L8                                Late
L8R                              Later
LBH                             Lets be honest
LOL                              Laughing out loud
LOL                              Lots of luck
LMAO                           Laughing my ass off. 
LMIRL                          Let's meet in real life.
LYLAS                          Love ya like a sister.

MIRL                            Meet in real life
MorF                            Male or Female 1
MOS                            Mom over shoulder
MTFBWU                     May the force be with you
MYOB                          Mind your own business.
MWA                            A kiss (the sound of "mwa.")

N2M                             Not too much.
NALOPKT                    Not a lot of people know this.
NBD                             No big deal
NE                               Any
NIH                              Not invented here.
NMU                            Not much, you?
Noob                           “Newbi” - someone who is a novice
NP                               No problem
NRN                             No response necessary

OIC                                            Oh, I see.
OMG                           Oh my God!
OTOH                         On the other hand.
OTP                            On the phone.
OWTTE                        Or words to that effect

P911                            Parent emergency
PAL                              Parents are listening.
PAW                            Parents are watching
PCM                            Please call me
PEBCAK                      Problem exists between chair and keyboard.
PEBKAC                      Problem exists between keyboard and chair.
PIR                              Parents in room.
PITA                            Pain in the ass.
PLBCAK                       Problem lies between chair and keyboard.
PLBKAC                       Problem lies between keyboard and chair.
PLZ                              Please
PMJI                            Pardon my jumping in.
POC                             Point of contact
POS                             Parent over shoulder.
POV                             Point of view
PRW                             Parents are watching.
PTG&LI                        Playing the game and loving it.

Q                                  Question

RL                                 Real life.
ROFL                            Rolling on the floor laughing.
RSN                              Real soon now
RUOK?                         Are you okay?

S2R                              Send to receive
SFETE                            Smiling from ear to ear
SIT                                Stay in touch
SOZ                              Sorry
STFU                            Shut the f*** up!
SUP                               What's up?
SYS See you soon

TANSTAAFL               There ain't no such thing as a free lunch.
TIA                                 Thanks in advance.
TMTH                           Too much to handle.
TTFN                           Ta, ta for now (goodbye).
TTT                              Thought that too.
TTYL                             Talk to you later.
TU                                 Thank you.
TY                                 Thank you.

U                                      You
U2                                   You too

W/E                                  Whatever
WB                                     Welcome back
WFM                                Works for me
WTG                                Way to go
WTH                                What the hell?
WU                                  What’s up?
WYGOWM                       Will you go out with me?
WYSIWYG                       What you see is what you get

XOXO                              Hugs and kisses

Y                                      Why
YT?                                   You there?
YW                                    You are welcome

ZZZ                                   Tired or bored