Subversion® is an open-source version control system. Subversion® allows developers to share their projects on repositories, where they are stored afterwards. A repository is very similar to a file server. The only difference is that it not only stores a copy of the file system, but also its previous states and changing history. Subversion® can be accessed over a network, so it provides the possibility for people to work on shared files and watch for changes made by other developers.
Subversion® home: http://subversion.tigris.org/