VGMusic
"21,000+ Game Music MIDI files." From NES to Xbox 360.
XOC - Super Mario World
"The complete soundtrack to Super Mario World, covered by one man using dozens of instruments. Roughly in game order, faithful to the originals, with some bizarre artistic license thrown around. A private hobby made public."
The Blindfolded Pianist: Mario Fantasy
A free collection of Super Mario Brothers and Final Fantasy songs on piano.
Marathon: The Trilogy Release
Marathon I, II, and III. Free!
Digital Press Sound Archives
Video game sounds from yesteryear.