Definitely must be the most misspelled word in the English language. I’ve seen blog after blog and comment section after comment section 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.
Sherry
Paul’s book–Literally, the Best Language Book Ever;
Everything Language and Grammar 