definately = 14,400,000 hits
definitely = 134,000,000 hits
"Definately" is one of the most common mistakes I see on message boards and forums. So I guess I shouldn't be surprised that it gets 10% as many hits as the correct spelling. Still ...
By the way, the first hit for "definately" is from the Urban Dictionary, which calls it "idiot-speak for 'definitely.'"
The first hit for "definitely" is a link explaining how to spell it.