Spy, as you may have guessed. I usually try to wreak as much havoc as possible by means such as stabbing fully charged medics (when applicable), killing weakened enemies with my revolver, aggravating bad/mediocre engies, getting a long chain of backstabs, causing a great deal of paranoia by spawncamping on certain maps -- but not the kind of spawn camping that inhibits the opposite team from leaving, because it's fairly difficult for one spy to do that, and picking off lone enemies as they begrudgingly walk from their spawn to the next CP.
Other than that, I do very little as spy.