The mr(1) command can checkout, update, or perform other actions on a set of repositories as if they were one combined respository. It supports any combination of subversion, git, cvs, mercurial, bzr, darcs, cvs, and fossil repositories, and support for other revision control systems can easily be added. (There are extensions adding support for unison and git-svn.)
It is extremely configurable via simple shell scripting. Some examples of things it can do include:
* Update a repository no more frequently than once every twelve hours.
* Run an arbitrary command before committing to a repository.
* When updating a git repository, pull from two different upstreams and merge the two together.
* Run several repository updates in parallel, greatly speeding up the update process.
* Remember actions that failed due to a laptop being offline, so they can be retried when it comes back online.