« GNU Arch is a revision control system, i.e. a system that makes it easy to track changes to computer files, mostly the source code to complex software systems. »
This software is a (or rather, one of the numerous) replacement for CVS. It is much more powerful, and makes it easy to work in a really open manner. You can create your own branch of a software managed by GNU Arch, commit to your branch with no special rights to be granted from the official branch, keep up with the original version, and make your changes available back when they are ready.
More information: http://wiki.gnuarch.org/