NFF stands for 'neutral file format'. There are (at least) two 3-d files with the NFF extension: Eric Haines's NFF scene description language and the Sense8 Corporation's neutral ASCII file format used by WorldToolKit.
File format information
- Description of the Haines NFF scene description language
- Description of the Sense8 WorldTookKit neutral ASCII file format
Sources of sample files
- NFF file anthology, Haines format (be sure to read the README at this site)