Although l33t speak is just a corrupted form of the English language, there are many phrases and words (spellings) that are unique to it.
0w|\| or 0wn3d - One of the most popular l33t words it is very loosely defined as beaten or can simply be an expression of awe, for example, 'I 0wn3d you' means 'I have beaten you in a very humiliating fashion', or '0wn4ge!' which means 'That was (or is) very nifty'.
w00t - Derived from 'hoot', this is defined as 'yay', it can be used, for example, upon victory or, possibly, the release and procurement of a new video card.
13wt - Treasure, good merchandise, possessions, a misspelling of loot. Most commonly referring to pirated software, items in a game or promotional giveaways.
h4x0r - Hacker, can be used for a real hacker or simply a very skillful person. This is the most common occurrence of the -0r clause.
ph33r - Fear, most commonly used in such phrases as, 'Ph33r m3!' or 'Ph33r |\/|y 1337 sk1llz!' It can also be written as, 'ph34r'.
sk1llz - Obviously derived from 'skill', referring to skill in some type of online game, programming or hacking. Many times used in conjunction with 'm4d'. As a general rule, if one has sk1llz, one is to be ph33r3d.
m4d - Mad, mostly used as a descriptive term meaning great, for example, 'h3s g0t m4d sk1llz'.
j00 - You, commonly used in such phrases as, 'j00 d34d f00'.
f00 - Fool, one who isn't very bright or skillful.
j0 - Yo, as in the greeting.
d00d - Dude; an expression of comrade, or just used to address a random person online.
sux0r - Sucks, as in '7h1s sux0r', one of the few common examples of the -0r clause.
l4m3r - Lamer, someone who is lame, someone who uses an unfair tactic or generally makes the things around him or her less fun.
n00b - Short for noobie, misspelling of newbie; someone who is new to something, or just not very good at it.