NOTE: This is a thread not for favorite videogame soundtracks themselves, but music which has been created external to the game itself!
(This can include tributes, remixes, covers, instrumentals, orchestrations, and other creative inspirations.)

Having written a few http://www.youtube.com/watch?v=3jG6iq4XLec&list=PL13E26C9FFCB63700 myself, I came to think about all the fan-made videogame-based music I enjoy listening to. One artist in particular comes to mind, Miracle of Sound, and the dozens of amazing tracks he's created. They are some of my favorite songs that I enjoy listening to endlessly - not only is he a great musician, the guy has an amazing talent for capturing the feel and stylistic cues of the games he creates for. The stand-out ones for me would be:

http://www.youtube.com/watch?v=TnbPGqEqTIQ (Fallout 3)
http://www.youtube.com/watch?v=oe-f25KmrdA (Deus Ex: Human Revolution)
http://www.youtube.com/watch?v=zV48aAF5AcM (Red Dead Redemption)
http://www.youtube.com/watch?v=mmViEzF8Hko (L.A. Noire)

Then there's the Metroid Metal music, by Stemage, which takes the classic Metroid soundtracks and - as you can imagine from the title - turns them into metal versions. Maybe not as original as Miracle of Sound above (although he does do some medleys which are rather creative) but still undeniably cool nonetheless.

Favorites would be:

http://www.youtube.com/watch?v=itZWNqox8xM
http://www.youtube.com/watch?v=Q7-1pJ-DJrk
http://www.youtube.com/watch?v=0IUtCLz_RIQ

And finally, before this post goes on too long, there are a few I've found for Skyrim that I particularly like: a version of the main theme by http://www.youtube.com/watch?v=BSLPH9d-jsI, and of course the already-familiar http://www.youtube.com/watch?v=wr-buV4tYOA by Malukah.

Without further ado, what are some of your favorites?

I'm a big fan of Miracle of Sound. Unfortunately he recently got Tinnitus so it's going to be a while before he can make music again. I hope it passes.