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, is written in C++, runs on Linux, Solaris, Mac OS X and other unixes, and Windows too.