/ = another form of “over to you” (from x/y as “x over y”)
/\/\/\ = A giggle or chuckle. On a MUD, this usually means 'earthquake fault'.
\ = lambda (used in discussing LISPy things)
= grin
= grinning, ducking, and running
AFAIAC = as far as I am concerned
AFAIK = as far as I know
AFK = away from keyboard
b4 = before
BBL = be back later
BCNU = be seeing you
BRB = be right back
BTW = by the way
BYE? = are you ready to unlink? (to end a talk-mode conversation; the other person types BYE to confirm))
CU l8er = see you later (mutant of CU l8tr)
CU l8tr = see you later
CUL = see you later
ENQ? = are you busy? (expects ACK or NAK in return)
FOAD = fuck off and die (use of this is generally OTT)
FOO? = are you there? (often used on unexpected links, meaning also “Sorry if I butted in")

FWIW = for what it's worth
FYA = for your amusement
FYI = for your information
GA = go ahead (used when two people have tried to type simultaneously; this cedes the right to type to the other)
GRMBL = grumble (expresses disquiet or disagreement)
HELLOP = hello? (an instance of the ‘-P’ convention)
HHOJ = ha ha only joking
HHOK = ha ha only kidding

HHOS = ha ha only serious
IIRC = if I recall correctly
IMHO = in my humble opinion
IRL = in real life)
JAM = just a minute (equivalent to SEC.... )
LOL = laughing out loud
MIN = same as JAM
MLA = passionate kissing (major lip action).
MORF = male or female?
NHOH = Never Heard of Him/Her
NIL = no
NP = no problem
O = over to you
OBTW = oh, by the way
OIC = oh, I see
OO = over and out
OTOH = on the other hand
OTT = over the top (excessive, uncalled for)
ppl = abbrev for “people”
R U THERE? = are you there?
rehi = hello again
ROTF = Rolling On The Floor
ROTF = rolling on the floor
ROTFL = rolling on the floor laughing
SEC = wait a second (sometimes written SEC... )
SYN = Are you busy? (expects ACK, SYNACK, or RST in return; based on the TCP/IP handshake sequence)
T = yes
TAN= = an aggressive male (tough as nails)
THX = thanks (mutant of TNX; clearly this comes in batches of 1138 (the Lucasian K)).
TIA = hanks In Advance
TNX 1.0E6 = thanks a million (humorous)
TNX = thanks
TNXE6 = another form of “thanks a million”
TTBOMK = to the best of my knowledge
TTFN = ta-ta for now
TTYL = talk to you later
TTYTT = to tell you the truth
UOK? = are you OK?
WRT = with regard to, or with respect to.
WTF = the universal interrogative particle; WTF knows what it means?
WTH = what the hell?
YHTBT = You Had To Be There
Source: Answers.com