l337
Created in 1980 by the original hackers of the Bulletin Board System. l337 is a variation os the word 'leet', which itself is derived from the word elite. Elite has been used in the past to designate a group of users as belonging to a higher social echelon than other users. The term Elite was gained from the classic game Elite.

The letter... could also be:
A - 4, /\, @, ^
B - 8, 13, %, ß
C - <, (, ©
D - |), I>, ?
E - 3, & 
F - (=, |=, ph
G - 6, &, (_+, 9
H - #, (-), /-\, \~\
I - 1, !, eye, 
J - </, (/
K - X, |<
L - 1, 7, |_
M - em, AA, (\/)
N - ^/, |\|
O - 0 (), oh
P - |*, |^, ?, 9
Q - 0_, ()_, <|
R - 2, |?, /2
S - 5, $, z, es
T - 7, +, 1
U - (_), v, L|
V - \/
W - \/\/, (n), \\'
X - ><, }{, )(
Y - j, `/
Z - 2, %, >_

A few terms taken from l337:
Pwn, omg, wtf, lyke, newb/n00b

Commonly used by: Gamers, computer geeks, newbs, idiots
"Dude, that's so... l337,"

0///6 d00)j00/2 z0 (_)nl337~!!111one!l0lzA!!!!!