Definitely has quickly become one of the most commonly misspelled words on the Internet. In fact, hardly a web page scrolls by where I don’t see it spelled definately.
Again, the correct spelling is definitely. Think of the other words contained within it, such as definite and finite (not definate and finate), or other words that are so closely related, such as definitive (not definative).
I am now definite that this should definitely put a definitive stop to misspelling it as definately.
Sherry
Paul’s book–Literally, the Best Language Book Ever;
Everything Language and Grammar 