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.
Codeville is a distributed version control system. It began with a novel idea for a merge algorithm and has grown from there. It is designed to be easy to use and scale from small personal projects to very large distributed ones.
Git is popular version control system designed to handle very large projects with speed and efficiency; it is used mainly for various open source projects, most notably the Linux kernel.