A Patch Queue Manager (PQM) is a program that sits on top of a repository (or set of repositories) and handles all kind of write access to it according to a set of policies like:

• who is allowed to commit (security)
• when access is opened (for release engineering tasks)
• which branches are open and to whom
• generate commit messages
• handle merge request from remote repositories (for synchronisation)
• and so on…

The nice thing is that a PQM can be VCS-neutral (although a centralised VCS doesn't need such a system).

The one I know of is the Bazaar one, inherited from Arch/tla days written by Robert Collins and now maintained through his work at Canonical. More information here. The repository can be found here: PQM repository in bzr format