Teemo - Build straight AP (mid) or AS/AP (top) and litter the place with mushrooms. You might not get kills, but you're better at racking up assists than Soraka. Stops people from running, helps with massive deeps in teamfights, and carries buy QSS because of Teemo. Infinite wards helps the team, too.
Master Yi - Not winning? Backdoor. It won't work often until you get the hang of it, but goddamn does it work wonders.
Karthus - Oh thank god! I'm safe! ... Son of a *DED* (You're required to learn how to farm a bit to get AP to make this guy's awesomeness work, though)