I always used to have problems with the word "definitely". When I was a kid I would often spell it "definetly" or "defenitly". Horrible, I know
One word that I notice a lot of other people have trouble with, but I don't, is "ridiculous". I always see people spelling it as "rediculous" and I can't even tell you how much that grates on my nerves. I have a friend who constantly spells it that way, even after being corrected numerous times, and she is a 4th year English major in university. Go figure.