I was just talking about this particular word with my nephew last week, I am surprised by how many people spell "definitely" incorrectly(and I am not talking about just on this site).
Also someone here on Pet Talk had posted this free online spell check site a while ago, and I have added it to my favorites.

Just a real quick story, when I first started this job it involved a writing a summary of the file for the next person to use in their job. I always thought of myself as a good speller. Imagine my embarrassment when after a week or two one of my co-workers came over and very politely offered me a dictionary to use. Moral of the story even if you think you know, it never hurts to check.