Standard ff R-code and C-code is under GPL-2 (see file COPYING that comes with your R distribution). 
Extended ff R-code and C-code is proprietary, currently we dual-license this only to supporters of package ff and friends.

NOTE that the .Rd documentation source is part of the .R files and thus copyright and licence apply to any exported documentation format as well.

C++ code by Daniel Adler is under the less restrictive ISC license version of free BSD (see file-headers).