An open sourced GIT system, Gitorious
Compared with Concurrent Versions System (CVS) and SVN, GIT is more distributed in its core, which means you do not have to have an account to work on a project. Branching the repository basically needs one command, making collaboration easy. The speed of pulling and pushing the code is fast. More explicit comparison is available here.
Gitorious is an open source project making start a GIT project practical and easy. Gitorious was initiated by Johan Sørensen in 2007 and then open sourced to public. The current active contributors to the project are Adolfo Sousa, Rodrigo Rosenfeld Rosas, Marius Mårnes Mathiesen, and more. Beside the opened source codes available for use, Gitorious also can be used to host projects. The current hosted projects are available here.
If you decide to make your own repository of GIT or use part of GIT core codes to make another distributed collaboration system, Gitorious is the way to go.