Here's a guy I only learned about recently but now can't get enough of.
British musician Blake Robinson has a little project called The Synthetic Orchestra, in which he creates orchestral compositions using various pieces of computer software and sampled instruments. Most of these songs are arrangements of videogame melodies, though he does branch out into TV and original pieces.
His music, like the Metal Gear medley above, is simple amazing. You can hit up his YouTube page and check out his whole catalog, which stretches back a few years. Understandably, his earliest stuff sounds nowhere near as authentic as the more recent pieces, but it's an impressive spread nonetheless.
I've embedded a few more songs after the jump, including Blake's rendition of "Sleeping Puppies" from the Me-Mow episode of Adventure Time. Just for kicks!
Gears of War - Main theme on Piano [YouTube]