Definitely must be the most misspelled word in the English language. I’ve seen blog after blog and comment list after comment list littered with this error.
When it is misspelled, however, at least it’s done in a consistent manner; it’s always definately. Perhaps it would make it easier to remember the correct spelling if we remember that definite has the word finite in it, and both words refer to having limits.
Sherry
Paul’s book–Literally, the Best Language Book Ever;
Everything Language and Grammar 