NES Sound Format
An 
.NSF (NES Sound Format) file is a sound data file, akin to 
SID from the 
Commodore, copied from a Nintendo Famicom or 
Nintendo Entertainment System (NES for short) game.  It can be played through 
WinAmp using various plugins, such as NES Amp.  NSF files are usually the entire soundtracks of the NES games, and may also contain sound effects..  
See also: Nintendo Entertainment System, list of NES games, SPC, SID, PSF