These always bother me, along with most of the previous mentioned:
Its (as in belonging to it, like, give the dog its food)
It's (It is)
Whose (possessive, as in, whose party is this?)
Who's (who is)
Who (used as a subject, like, who are you, who did this)
Whom (used as an object, like, to whom is this for, you said that to whom)