2 - to, or too
c - see
k - O.K./okay
o - oh
q - question
r - are/our
u - you
y - why
Double letter abbreviations:

am - above mentioned
cu - see you
ga - go ahead (which originated with Morse Code on the telegraph and later entered usage on IRC)
gg - good game or gotta go
g2g - got to go
jc - just chillin'
kl - cool
kk - ok cool (often to emphasize k)
nm - nothing much or nevermind
np - no problem
rl - real life
tg - thats gooey
ty - thank you
ur - your/you're
wb - welcome back
yw - you're welcome
Abbreviations using three or more letters:

aamof - as a matter of fact
afaik - as far as I know
afk - away from keyboard/a free kill
agg - a term meaning good or awesome usually referring to paintball
*poof* definitely stopped watching the monitor now (or even turned it off).
atm - at the moment
asl - age/sex/location
brb - be right back
brt - be right there
bbiab - be back in a bit
bbl - be back later
bfn - bye for now
btw - by the way
byob - Bring your own booze/beer/beverage/boxing gloves/bombs
cba - can't be assed/asked
cbb - can't be bothered
cbf - can't be fucked
cuz - because
cwus - converse with you soon
cya - See you (later)
ftl - For the lose
ftw - For the win
fyi - for your information
g2g - got to go (or "good to go" as distinct from "gtg" for "got to go")
gtfo - get the fuck out
gtg - got to go
hax - hacks/hacking
hxc - hard core
iaal - I am actually laughing
idc - I don't care
idk - I don't know
iirc - If I remember/recall correctly
ily - I love you
imba - Imbalanced or over-powered (usually in a gaming context)
ime - in my experience
imho - in my humble/honest opinion
imo - In my opinion
irl - in real life
l8r - later
lmao - laughing my ass off
lmfao - laughing my fucking ass off
LDR - Long-distance relationship
leet - elite
lol - laughing out loud [a user says this when something is funny, though it has become somewhat of a cliché
Lawl(z) - "lol" when spoken aloud. Also used to express onces literacy.
Lollerskates - Footwear shoved violently into the mouth of he who uses "lol" far too much
nsfw - Not suitable/safe for work
NITSKW - Not in the scene kid way (usually follows ILY... CARA!! *waves*)
omg - oh my goodness/gosh/god
ZOMG - a way to make fun of "scene" kids who overly use the term "OMG". Similar to 'pwn' in that it stems from the proximity of the shift and Z keys.
O RLY? - oh really?
YA RLY - yeah, really.
NO WAI!!! - no way!
YES WAI!!! - yes way!
otp - on the phone
paw - parents are watching
peps - people
ppl - people
pr0n - porn
ROFL - Rolling on floor laughing
LQTM - Laughing quietly to myself
ROTFLOL - Rolling on the floor laughing out loud
ROFLcopter - a helicopter made out of ROFL's, used to replace "ROFL" in certain cases.
ROFLtrain - A steam train with a whistle that goes "woot-woot" and pistons that "rofl" as they move.
ROFLcakes - Giant, 100-layered cakes that are transported on a cart who's wheels scream "rofl" as they move
Roffle - "rofl" when spoken aloud. Also used to express/prove one's literacy.
ROTFL - Rolling on the floor laughing
ROFLmania - Twist on wrestlemania, but with rolling on the floor laughing instead of wrestling.
s2r - send to receive. used to signal offer to exchange pictures.
SAL - Smiling A Little
sif - As if
SLAP - Sounds like a party
SRSLY - Seirously
STBE - Soon-to-be-ex, as in ex-husband or ex-wife
moo? what?
meow? what?
kthx - okay? thanks! (sarcastically)
kthxbye (also sometimes kthxbai) - okay, thanks, goodbye. Used like kthx, but additionally closing (or trying to close) the discussion and/or thread.
stfu - shut the fuck up
tbh - to be honest
tbqh - to be quite honest
thx thanks
TIA - Thanks in advance
tldr (also sometimes tl;dr) - too long; didn't read. Used in discussions when the reader refuses to read the other person's opinion because it is too long.
ttfn - "Ta ta for now" (see you later)
ttiuwp - "This thread is useless without pictures" - Usually spammed when females are mentioned on forums.
ttt - to the top, used in forums ordered by last post
ttyl - talk to you later [originally developed by the deaf on teletype machines]
ttys - talk to you soon [similar to the above]
tyt - take your time
w/e - whatever
w/o - without
fu - fuck you
pgm - pro gamers
i'z - i is (ebonics for "i am")
ftl - For the Loss/lose
mlk - malakas -greek word- meaning masturbator or a stupid person
kewl - cool or awesome
own- To pose superiority above the victim or competitor beaten badly.
pwn- Leetspeak version of the word own, used in the sense of "I own you".
pwnt- Leetspeak version of the word own, used in the sense of "I owned you".
ke - Ok
WTF- "What the Fuck" - An expression of incredulity. also "Why the Fuck", "Who the Fuck" or similar replacements of the W
WTH- "What the hell"
owned- used mostly in online games when defeting an oponent.
p4ned- another way to spell owned used mostly in online games when defeating an oponent.
?ber- ?ber is a gaming term for a player or object that is considered superior, originating from german meaning "greater" or something of the sort
ZOMFGHIJKLOL- "Oh My Fucking God, Homie; I'm Just Kidding; Laugh Out Loud." a slight parody on the term "OMG" and it's many alterations.
Abbreviations using charactors other than letters, see [emoticon] for more:

:) - means happy
- means sad
:I - means not feeling any emotion at the time
x_x - dead, exhausted or knocked out; giving up, lost, confused
0_o - shocked, appalled, in a state of disbelief