A rather comprehensive list of common abbreviations used online.

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ADNAny Day Now
AFKAway From the Keyboard
AFNthat's All For Now
a/s/lAge/Sex/Location - (used to ask a chatter their personal information)
AVAvatar - Graphical representation of a person chatting.
BBLBe Back Later
BFNBye For Now
BRBest Regards
BRBBe Right Back
BRHBe Right Here
BTWBy The Way
CUSee You
CUL8RSee You Later
CUOLSee You On Line
CYASee You - (as in "See you later)
DISDid I Say
DL -or d/lDownload... or downloading.
EGEvil Grin
EOTEnd Of Thread (meaning end of discussion)
F2FFace To Face
FAQFrequently Asked Question
FISHFirst In Still Here (someone who is on line TOO much)
FOCLFalling Off Chair - Laughing
FUBAR"Fouled" Up Beyond All Repair / Recognition
FUDFear, Uncertainty, and Doubt
FWIWFor What It's Worth
FYIFor Your Information
GAGo Ahead
GALGet A Life
gestGesture - a combination text / audio / video file played in VP Chat rooms
GRGotta Run
GTRMGoing To Read Mail (leaving chat room to check email)
IACIn Any Case
I ANAL (or IANAL)I'm Am Not A Lawyer (expect an uninformed opinion)
ICI See
ILU or ILYI Love You
IMHOIn My Humble Opinion (IMO = In My Opinion)
IOHI'm Out of Here
IOWIn Other Words
IRLIn Real Life
JICJust In Case
K -or- kkO.K.
JK -or- j/kJust Kidding
L8RLater
LMAO -or- LMFAOLaughing My (F'ing) Ass Off
LOLLaughing Out Loud
mugPerson chatting in VP rooms using their browser (IE, Netscape)... instead of the download client program.
NFWNo Feasible (or F***ing) Way
NPNo Problem
NRNNo Response Necessary
OICOh, I See
OJ or OKOnly Joking or Only Kidding
OTOHOn The Other Hand ...
OTMHOff the Top of My Head ...
OTWOn The Way (used when someone is sending a file)
PANSPretty Awesome New Stuff (often referring to computer technology)
PCMCIAPersonal Computer Memory Cards International Association
PCMCIAPeople Can't Master Computer Industry Acronyms (slang)
PEBCAKProblem Exists Between Chair And Keyboard
PITAPain In The Ass
POTSPlain Old Telephone Service
ROFLRolling On Floor, Laughing
ROTFRolling On The Floor - (refers to someone laughing very hard)
ROTFLMAORolling On The Floor, Laughing My Ass Off
RSNReal Soon Now
RTFMRead The "Flippin" Manual, (esponse to beginner question on net, chat, newsgroups, etc.)
SEGShit Eating Grin
SNAFUSituation Normal, All "Fouled" Up
SOLShit Out Of Luck
SOTAState Of The Art (latest technology)
SYSincerely Yours
SYLSee You Later
TAFNThat's All For Now
TFHThread From Hell (a topic or discussion that won't stop - esp. newsgroups)
TGIFThank Goodness It's Friday
THXThanks!
TNTTil Next Time
TPTBThe Powers That Be
TTFNTa-Ta For Now
TTYLTalk To You Later
TYThank You
VPPHVirtual Places Page Host. (Others now using software on their own servers to talk to VP clients in the same manner that Excite once did)
VNVery Nice
WCWelcome... as in "You are Welcome"
WEUWhat's Eating You?
WFMWorks For Me
WT?What The ...? or Who the ...?
WTGP?Want To Go Private? (move to a private chat room)
YWYou're Welcome