1337-speak replacement for "you". Usage is correct when used to replace the "-ed" of a past tense verb followed by "you". Commonly misused as a simple replacement for "you" regardless of the preceding word.
I h4x0rd you -> I h4x0r j00
I pwnd you -> I pwn j00
However, the following examples are both incorrect:
I pwn you (present tense pwn) -> I pwn j00 (tense changes)
I pwnd you -> I pwnd j00 (redundant)