'[t[1]6502 machine code for the Nintendo Entertaiment System's 2a03 sound chip. NSF stands for '[o]NES'[/o] 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 '[l[http://rainwarrior.thenoos.net/projects/nsfplay/index.html[NSFPlay] and '[l[http://www.zophar.net/utilities/nsf/virtuansf.html[VirtuaNSF]. There is a more comprehensive list with compatibility info '[l[http://famitracker.com/wiki/index.php?title=List_of_NSF_players[here]. 

NSF file format specs available from Kevin Horton at '[l[http://kevtris.org/nes/nsfspec.txt[CLICK HERE]