Quote Originally Posted by ReDSanchous
I've always thought that who and whom are interchangeable. seems like whom is kind of formal. I hardly ever use it.
Who, whoever, etc are nominative. Whom, whomever, etc are non-nominative. So its "Whoever called me STFU!" lol, but its "Call whomever you want" or "To whom it may concern" but not "To who it may concern."

But whom is becoming archaic, especially in casual conversation. In most cases you can use who in place of whom and it actually sounds more normal.