Darcs is a free, open source source code management system.

Every user has access to the full command set, removing boundaries between server and client or committer and non-committers.

Darcs is easy to learn and efficient to use because it asks you questions in response to simple commands, giving you choices in your work flow. You can choose to record one change in a file, while ignoring another. As you update from upstream, you can review each patch name, even the full "diff" for interesting patches.