Slang with 'Numbers'

10-4 — "OK, Acknowledgement", as in the CB Radio 10-codes.
12B — "Wannabe"
10q — "Thank you"
10x — "Thanks"
1337 — "Leet", short for elite, used ironically.
20 — "location"; from CB Radio jargon
224 — "today, tomorrow, forever"
2B — "to be"
2B||!2B — "to be or not to be"; from the C programming language logic operations "OR" ("||") and "NOT" ("!").
2m — "tomorrow"
2TM — "to the max"
31337 — "elite", used ironically
3Q or 3U — "thank you" (Japanese pronunciation of "thank you" as sankyū, and of "3" as san.)
4 — "for"
404; "Couldn't find it" (from the HTTP 404 "Error: Resource not found" response code)
4649 — "pleased to meet you", "hi" (from Japanese "yoroshiku", the four numbers can be pronounced yo-ro-shi-ku)
46 — "pleased to meet you" "hi" (short version of 4649)
4ever, 4eva — "forever"
5/5 - "five by five"
807, 8075 — "BOT(s)"; a person who demonstrates an extreme lack of knowledge or common sense. Originally deriving from forum or instant message bots, which often respond to pre-programmed keywords and commonly-used phrases humorously, without really making any coherent sense
88, 881 or 886 — "bye bye" (Chinese pronunciation). In some racist chat rooms it means "heil Hitler", because the letter "H" comes eighth in most Roman alphabets
9494 — "that's it", "that's right", "you see?" (Chinese pronunciation)
1AB — Wannabe—A person who wants to act/be another person (One-A-Be; 1-A-B)

Slang with 'Characters'

(>")> / (>'.')> — 'Kirby'
^H — Signifies pressing the backspace key. "What an idio^H^H^H^Hinsightful comment!" (Stems from the use of Ctrl + H as backspace in some Unix shells)
^W — Similarly, this means erasing the last word. "I mean, who really gives a^W^W cares?"
> — Greater than (as in "Linux > Windows" or "Windows > Linux"). Use of an asterisk (*) in this context sometimes denotes a wildcard, for example,. "John > *" means "John is greater than all". (Depending on context, the reader may substitute the word "owns" in place of the phrase "is greater than". Another possible interpretation here may state: "John is better than all")
^ ^ or ^__^ or ^^ — an emoticon signifying happiness, especially used by anime fans. "Chuckle marks"
>_< or >< — an emoticon signifying frustration or embarassment
>_> — an emoticon used to indicate sarcasm or suspicion, sometimes used with <_< to indicate shifty eyes
T__T — Crying (often implying crying in a humorous way)
>^.^< — cat face or a face that has crinkled up with laughter
\,,/. — Throwing the horns
x__x — dead, unconscious, etc.
^__~ — wink, reluctant
@_@ — symbol for a very tired person trying to stay awake, going crazy, dizzy.
$_$ - Money hungry
:-) smiley face (as with many things in computerspeak, the reader should view this emoticon sideways)
;-) winking
-__- — disgusted, annoyed, etc.
^o^ — laughing, cheerful, joyous, similar to ^^
^^__^^ — Angel
(^_^) — Happy face
[*_*] — Scared, in awe
><)))*> — somethings fishy
=^__^= — Cat face, blushing
-^__^- — Blushing
=_= - tired, with bags under eyes
<^__^;> — symbol for embarrassment commonly used by anime fans
_|_ — an offensive emoticon signifying the middle finger
_W_ — "What ever", "I do not care"
‹^› — another form of the middle finger
..i.. — yet another form of the middle finger (often seen in pairs)
<3 — "love"; ASCII art for a heart, the less-than symbol and the three resemble a heart tilted on its side.
</3 — "hate"; ASCII art for a broken heart, similar to <3.
== — agreement; from the C equality operator; means "me too" or "what he said"
!= — disagreement; from the C inequality operator; means "is not (necessarily) the same as" Sometimes also written as =/= as in mathematical notation.
++ — Used after the manner of the C operator to indicate something "one greater" than itself, for example: "Awesome++"; also used similarly to the "doubleplus" prefix in Newspeak.
-- — Opposite of ++, indicating something "one less" than itself.
@ — "at". Some users go so far as to replace all "at" sounds in words with @, so "fat" becomes "f@"; "matter" = "m@er" and so forth, although this occurs relatively rarely.
* (asterisk) — applied around a word or phrase to indicate emphasis or italics, or just typing in bold face. Singly, in chat, in front of a word or phrase at the beginning of a line means a correction to one's own previous typo. It recently started being used for an action. E.G *Jumps up and down*
_ (underscore) — similar to asterisk, but means underlining of a word or phrase. For example, "Do you _really_ think that?"
/ (forward slash) — indicates italicized text, similar to the underscore for underlined text
-0_o- — an emotion expressing confusion, shock, drunkenness or a "high" state
0.o — an emoticon expressing confusion or shock, signifying the user saying "WHAT!?!?"; also designating a drunken or high state
Ô_ô — an emotion expressing confusion, shock, drunkenness or a "high" state
9_9 — rolling eyes, used to express annoyance
:O — "Oh" Emoticon
\o/ or \^o^/ — the 'yayman', representing the pose that someone with something to celebrate might adopt. In contexts such as forums, where small images replace emoticons, some type of happy emoticon image often replaces the 'o'.
\ob — similar to the yayman, the 'b' represents a thumbs-up.
/o\ — more or less the opposite of the yayman, here the figure puts his hands on his head: 'Oh no!'
!!!111!!!11!!!11 - To make fun of noobs/newbs/n00bs on the net, or to also make fun of AOL people, put at end of sentence.
!!!!!!!!11!!!11!!!11!!One - See above
!?!? or ?!?! or !? or ?! — showing surprise and confusion; this basically expresses anger, shock/surprise, or confusion, and rarely speechlessness
ajkaf;lsdjfjdskl;afj (any random string of lowercase characters, usually from the keys in home position on the keyboard) — hysterical laughter
{}==[]:::::::::::::::> - sword
>>>-------------> - arrow
¬_¬ - showing suspiscion or distrust

Slang with letter 'Z'

z00t — Also "zoot"—bastardization of "w00t"
0day — Software or media that is illegally obtained before or on the day of the official release.
zig — "Cigarette".
ZOMG — Bastardization of "OMG", meaning "oh my God". Derived from the accidental use of the "z" key when the typist intended to use the Shift function. Pronounced "zoe my God", "zee oh em gee", or "zed oh em gee" (the last two varying on the regional pronunciation of the name of the letter Z).