zOMG is a varient of the all-too-popular acronym "OMG", meaning "Oh My God".
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.
"zOMG! you r teh winz!!one!!eleven!"