The "z" was originally a mistake while attempting to hit the shift key with the left hand, and type "OMG"
Also used in all-caps, 'ZOMG' is generally used in a sarcastic manner, more often than not a humiliating fasion. It is also used as a device for stating the obvious.
It was used by some gamers for a time in 2004. In 2006, its usage became widespread.
It was picked up and used as a humorous way of saying omg and making fun of internet slang in general do to its funny pronunciation of "Zoh-my-God!".