Piston is a utility that eases vendor branch management. This is
similar to svn:externals, except you have a local copy of the files,
which you can modify at will. As long as the changes are mergeable,
you should have no problems.

Piston has a similar purpose than svnmerge.py which you can find in
the contrib/client-side folder of the main Subversion. The main
difference is that Piston is designed to work with remote
repositories.

WWW: http://francois.github.com/piston