[khedoros1]

These are all variants of NSF players. When looking for them, my requirement was that they generate audio with an actual NES CPU.

This one I think is especially cool. Instructions to build a cartridge to run on an unmodified NES: https://web.archive.org/web/20170319202533/www.nullsleep.com...

And these two both use an NES (or Famiclone) CPU, along with some other hardware:

http://kevtris.org/Projects/hardnes/

http://forums.nesdev.com/viewtopic.php?t=5957&view=next

NSF is a cool format, itself. It's basically a specialized ROM format for encoding game music and sound effects for playback, usually in an emulator. And since it relies on actual CPU code rather than recorded register writes, it can include loops and such programmatically.