http://piston.rubyforge.org/
From the README:
Piston is a utility that enables merge tracking of remoterepositories. This is similar to svn:externals, except you have alocal copy of the files, which you can modify at will. As long as thechanges are mergeable, you should have no problems.
Piston has a similar purpose than svnmerge.py which you can find inthe contrib/client-side folder of the main Subversion. The maindifference is that Piston is designed to work with remoterepositories. You should also investigate if SVK would be a better fitfor you.
From Wikipedia's Piston:
    In general, a piston is a sliding plug that fits closely insidethe bore of a cylinder.
    Its purpose is either to change the volume enclosed by thecylinder, or to exert a force on a fluid inside the cylinder.
For Piston, we retain the second meaning, to exert a force on a fluidinside the cylinder. In our case, the fluid is the code copied fromthe remote repository.
= Installation
 $ gem install piston
Piston Home Page: http://piston.rubyforge.org/
Have a nice day !-- Franois Beausoleilhttp://blog.teksol.info/