6502 machine code for the Nintendo Entertaiment System's 2a03 sound chip. NSF stands for NES Sound Format, but this is frequently mistaken as Nintendo Sound Format.
A set of tools create .NSF files like PPMCK, NerdTracker and Famitracker.
Examples of tools that can play .NSF files include NSFPlay and VirtuaNSF. There is a more comprehensive list with compatibility info here.
NSF file format specs available from Kevin Horton at CLICK HERE
