At first glance you'd assume the only thing Matt Mulholland has mastered is Portal, but this Cousin Larry-looking everyman is actually quite the talented musician, one who has mastered the art of the YouTube a capella multitrack, and in doing so has developed quite the cult following.
Matt really came into his internet own with his tremendous Ghostbusters mutlitrack that I defy you not to appreciate. Before that he dabbled with comedy sketches and pop music covers, but dude must have realized toying with easily identifiable pop culture should be his wheelhouse because he's made it his bread and butter ever since. All his best stuff to follow, including: The Ghostbusters multitrack, "Big Willie Style" A Capella, "Harder Better Faster Stronger," and the just released (and already viral) Matrix Lobby Scene A Capella that made me forget about the The Matrix sequels and Speed Racer.
His YouTube channel is also chock full of d*ck joke vids and a recorder interpretation of "My Heart Will Go On," but I'll leave those up to you to check out.