A

ADN - Any Day Now
AFAIK - As Far As I Know
AFK - Away From Keyboard
AAIK - As If I Care
AKA - Also Known As
ASAP - As Soon As Possible
AYF - All Your Fault

B

BAK - Back At Keyboard
BAMF - Bad Arsed Mother F******
B/C - Because
BBL - Be Back Later
BIL - Brother In Law
BOC - But Of Course
BOT - Back On Topic
BRB - Be right back 
BTW - By The Way

C

COTFLGOHAHA - Crawling on the Floor Laughing Guts Out and Having a Heart Attack
CU - See You
CUL8R - See You Later
CYA - Cover Your Arse

D 

DD - Darling Daughter
DH - Darling Husband
DP - Darling Partner
DS - Darling Son

E

ETA - Estimated Time of Arrival
EDD - Estimated Date of Delivery

F

FAQ - Frequently Asked Questions
FC - Fingers Crossed
FIL - Father In Law
FWIW - For What It's Worth
FYI - For Your Information

G

G2G - Got To Go
G - Grin
GMTA - Great Minds Think Alike
GR&D - Grinning, Running & Ducking
GTG - Got To Go
GTRM - Going To Read Mail

H

HSIK - How Should I know
HTH - Happy To Help
HTH - Hope This Helps
HTML - Hyper Text Markup Language

I

IAE - In Any Event
IBK - Idiot Behind Keyboard
IIRC - If I Recall Correctly
IITYWTMWYBMAD - If I Tell You What This Means, Will You Buy Me A Drink
ILF - I'll Look Forward
IMCO - In My Considered Opinion
IMHO - In My Humble Opinion
IMNSHO - In My Not So Humble Opinion
IMO - In My Opinion
IOW - In Other Words
IRC - Internet Relay Chat
IRL - In Real Life
IYKWIM - If You Know What I Mean

J

JFYI - Just For Your Information
JIC - Just In Case
J/K - Joke

L

L8R - Later
LMAO - Laughing My Arse Off
LMKOAP - Let Me Know Of Any Problems
LOL - Laughing Out Loud

M

MIL - Mother In Law
MMP - More Money Please
ML - Mailing List

N

NBD - No Big Deal 
NFW - No F****** Way
NOYB - None of Your Business

O

OTOH - On The Other Hand

P

PMSL - Pissing Myself Laughing
POV - Point Of View

R

ROTF - Rolling On the Floor
ROTFLMAO - Rolling On The Floor Laughing My Arse Off

S

S - Smile
SIL - Sister In Law
SO - Significant Other

T

TAFN - That's All For Now
TIA - Thanks In Advance
TNTL - Trying Not To Laugh
TTFN - Ta Ta For Now
TTYL - Talk To You Later
TY - Thankyou

U

URL - Uniform Resource Locator (i.e. a web link)

V

VBG - Very Big Grin
VEG - Very Evil Grin

W

WB - Welcome Back
WRT - With Respect To/With Regard To
WTF - What The F***
WYLABOCTGWTR - Would You Like a Bowl of Cream to go with That Remark?
WYLASOMWTC - Would You Like a Saucer of Milk with that Comment?
WYSIAWYG - What You See is Almost What You Get
WYSIWYG - What You See Is What You Get
WYSINWNWYG - What You See Is No Where Near What You Get
WYSINWYG - What You See Is NOT What You Get
WYPIWYG - What You Printed Is What You've Got
WYTYSYDG -  What You Thought You Saw, You Didn't Get

X

Y 

YW - You're Welcome

Z