Speaking of Rihanna, here's one of Disturbia:
YouTube - Rihanna - Disturbia (acoustic) on iTunes

Their cover of Viva La Vida is also really nice.

The audio is crappy, but I still think this guy is one hell of a singer. Anthem from Chess.
YouTube - Anthem (Chess) Joshua Bryan Page

This cover is just the best cover in the world. Could listen to it forever. Romeo & Juliet:
YouTube - Romeo and Juliet (Cover)

I intend to marry this girl. covering She by Green Day:
YouTube - She cover- acoustic