EM Culture: OverClocked Remix
Since 1999, OverClocked Remix has been a thriving network for preserving, appreciating, and interpreting video game music. It's the only place you'll find Mario Jazz, Zelda orchestral, and Mega Man metal. They even worked with Capcom on Super Street Fighter II Turbo HD Remix. There are all sorts of freely released albums and singles you can download.
UPDATE: They JUST released a new album called HEROES VS. VILLAINS. Click here to download.
Here is Koopa Castle remixed by zircon:
And here is a sample of analoq and Hemophiliac's remix of Startropics which you can download for free here: http://ocremix.org/remix/OCR01755/