Monotone is a free distributed version control system. It provides a simple, 
single-file transactional version store, with fully disconnected operation and
an efficient peer-to-peer synchronization protocol. It understands 
history-sensitive merging, lightweight branches, integrated code review and 3rd
party testing. It uses cryptographic version naming and client-side RSA 
certificates. It has good internationalization support, has no external 
dependencies, runs on linux, solaris, OSX, windows, and other unixes, 
and is licensed under the GNU GPL.