Depending on where you are online, you may encounter different sets of shorthand. While no one set of acronyms is necessarily complete, this list should cover most any abbreviation you are likely to come across:

2U2   

 To You, Too

AAMOF  

 As A Matter Of Fact

AFAIK  

 As Far As I Know

AFAIC  

 As Far As I'm Concerned

AFAICT  

 As Far As I Can Tell

AFK  

 Away From Keyboard

ASAP  

 As Soon As Possible

BAK 

 Back At Keyboard

BBL 

 Be Back Later

BITMT 

 But In The Meantime

BOT 

 Back On Topic

BRB 

 Be Right Back

BTW 

 By the way

C4N  

 Ciao For Now

CRS  

 Can't Remember "Stuff"

CU  

 See You

CUL(8R)  

 See You Later

CWOT  

 Complete Waste Of Time

CYA  

 See Ya

DITYID 

 Did I Tell You I'm Distressed?

DIY 

 Do It Yourself

EOD  

 End Of Discussion

EZ  

 Easy

F2F 

 Face To Face

FAQ 

 Frequently Asked Questions

FBOW 

 For Better Or Worse

FOAF 

 Friend Of A Friend

FOCL 

 Falling Off Chair Laughing

FWIW 

 For What It's Worth

FYA 

 For Your Amusement

FYI 

 For Your Information

GA  

 Go Ahead

GAL  

 Get A Life

GBTW  

 Get Back To Work

GFC  

 Going For Coffee

GFETE  

 Grinning From Ear To Ear

GMTA  

 Great minds think alike

GR&D  

 Grinning, Running & Ducking

GTG  

 Got To Go

GTGTTBR  

 Got To Go To The Bathroom

GTRM  

 Going To Read Mail

HAND 

 Have A Nice Day

HHOK 

 Ha Ha Only Kidding

HTH 

 Hope This Helps

IAC  

 In Any Case

IAE  

 In Any Event

IC  

 I See

IDGI 

 I Don't Get It

IMCO 

 In My Considered Opinion

IMHO 

 In my humble opinion

IMNSHO 

 in My Not So Humble Opinion

IMO 

 In My Opinion

IMPE 

 In My Previous/Personal Experience

IMVHO 

 In My Very Humble Opinion

IOTTMCO 

 Intuitively Obvious To The Most Casual Observer

IOW 

 In Other Words

IRL 

 In Real Life

ISP 

 Internet Service Provider

IYKWIM 

 If You Know What I Mean

JIC 

  Just In Case

J/K 

  Just kidding

KISS 

  Keep It Simple Stupid

L8TR 

  Later

LD 

 Later dude

LOL 

 Laughing Out Loud

LTNS 

 Long Time No See

MorF 

 Male or Female, or person who asks that question

MTCW 

 My Two Cents Worth

NRN 

 No Reply Necessary

ONNA 

 Oh No, Not Again!

OTOH 

 On The Other Hand 

OTTOMH 

Off the top of my head

OIC 

 Oh I See

OTF 

 On The Floor

OLL 

 Online Love

PCMCIA 

 People Can't Memorize Computer Industry Acronyms

PLS 

 Please

PU 

 That Stinks!

REHI 

 Hello Again (re-Hi!)

ROFL 

 Rolling On Floor Laughing

ROTF 

 Rolling On The Floor

ROTFL 

 Rolling On The Floor Laughing

RSN 

 Real Soon Now

RTDox 

 Read The Documentation/Directions

RTFM 

 Read The Frickin' Manual

RUOK 

 Are You OK?

SNAFU 

 Situation Normal; All Fouled Up

SO 

 Significant Other

SOL 

 Smiling Out Loud (or You're Out of Luck)

TANSTAAFL 

 There Ain't No Such Thing As A Free Lunch

TAFN 

 That's All For Now

TEOTWAWKI 

The End Of The World As We Know It

THX 

 Thanks

TIA 

 Thanks In Advance

TLK2UL8R 

 Talk to you later

TMK 

 To My Knowledge

TOS 

 Terms Of Service

TPTB 

 The Powers That Be

TSWC 

 Tell Someone Who Cares

TTBOMK 

 To The Best Of My Knowledge

TTFN 

 Ta-Ta For Now

TTYL(8R) 

 Talk To You Later

TWIMC 

 To Whom It May Concern

Txs 

 Thanks

URL 

 Web Page Address

w/b 

 Welcome Back

w/o 

 Without

WRT 

 With Regard To

WTG 

 Way To Go

WU? 

 What's Up?

WWW 

 World Wide Web

WYSIWYG 

 What You See Is What You Get

Y2K 

 Year 2000

YGIAGAM 

 Your Guess Is As Good As Mine

YGWYPF 

 You Get What You Pay For

YMMV 

 Your Mileage May Vary

ZZZ 

 Sleeping