Ted's entry some time ago mentioning music from MegaMan got me thinking about some of the better game-inspired compositions I've heard recently, and I've been listening to one artist in particular nearly exclusively for the past week and a half. His name is Stephan Ajax, better known as AmIEvil in the ocremix.org community. He's done quite a few pieces and posted them on www.ocremix.org. Here's links to a copule of my favorites, but pretty much everything he's done is genius and rather addictive.
Let there be light, based on the Dust Man theme from MegaMan 4
River of Time, based on music from The Legend of Zelda: Ocarina of Time
Red XIII Redux, based on the Cosmo Canyon theme from Final Fantasy VII
Monkey Island Theme, this one's obvious
Zelda Jazz, based on the original Zelda theme
Samus Strut, based on music from the original Metroid game
The rest of his works are really good, these are just some of my absolute favorites.