l33t
As the Internet was coming into its own, during the early 1980s, hackers that didn't want their websites, newsgroups, etc, to be picked up in a simple keyword search began using numbers to replace certain letters (mostly vowels) such as A = 4 or E = 3.

At this point, l33t speak was only known to a select few and only used when necessary. However, in 1994, id Software began to add Internet connectivity to Doom and Doom II, leading to a revolution in PC gaming and also to the rise of l33t speak.

As with any type of competition, 'smack' talk became prevalent in online gaming. Phrases such as 'I am elite' became common place, and somewhere down the line l33t speak crept in, reforming the phrase into '1 4m 3l1t3' in order to demonstrate that the speaker was a hacker and someone to be feared. It was further exaggerated by purposeful bad spelling and eventually wound up as something like this, '1 4m 3l33t!' and simplified to, '1 4m 133t'. Hence the name 'l33t speak'.

l33t speak was still relatively unknown to the Internet masses at this point. It had almost disappeared from gaming entirely. However, a very popular web comic called Megatokyo brought l33t speak into mainstream with its infamous speak l33t? comic. These days l33t speak is very well known to the hardcore Internet community (especially gamers).

As you've already seen, basic l33t is just replacing vowels with numbers:

A = 4
E = 3
I = 1
O = 0
However l33t is a very flexible 'language' and you can go from this very basic l33t, to ultra 1337 by being creative; a few examples:

O = ()
U = |_|
T = 7
D = |)
W = \/\/
S = $
There is no agreed-upon way to write l33t, so it's up to you whether or not to go with light l33t, medium 1337, hard |_337 or even ultra |_33¯|¯. In this entry, 'l33t' is used except when referring to medium, hard or ultra '1337'.

The -0r clause can be used in place of -ed or -er, for example '1 0wnz0r!' (most commonly seen in ultra 1337 as '1 0\/\/|\|z0r!') rather than '1 0wn3d'. However it can also be used like this, '1 4m l33t h4x0r!' (in ultra 1337, '1 4|\/| |_337 |-|4x0r!') rather than, '1 4m 4 133t h4ck3r!'.

As a general rule, in l33t, rather than use 's' to make something plural, a 'z' is used instead. Also 'f' is normally changed to 'ph'. The short 'u' is often changed to '00', as well.

Punctuation is usually left out with greater consistency as the level of l33t increases. Full stops are left out entirely because, when you are chatting online, where you would normally put a full stop you just hit 'enter' to send the message and keep typing. Exclamation marks are used quite a bit (and usually quite a number at once), mostly because l33t is most commonly used in expressions of pleasure. Commas are generally left out in hard and ultra 1337, and are uncommon in medium 1337.

Recently after the dawn of MMORPGs such as World of Warcraft as a popular form of online gaming, l33t speak has become an epidemic to the point of burnout.  Most l33t speak is generally considered a sign of immaturity or is done derisively and tounge in cheek.  
immature:
some "l33t" loser "OMGZ0rz WTF n00b u d0n7 5|*34k l33t!!1 u r n0t 4 h4><0rz!!!"
everyone else "jeez, what a loser."

toungue in cheek:
friend 1 "oops i forgot to take off my caps lock"
friend 2 "OMGZ CAPS U 5|_|x n00b!!1one  =)"
friend 1 "haha.  gosh i must be a noob =p"
