Source Code Manager

Hope this is the right list for this. I have some open source code that a client of mine wants to use on his site. The contributor of this code is very good about updating and providing new releases of his code a couple of times a year. I am also generating some fixes (not many) and new modules for this client. I would like to be able to keep my code separate from the contributors code, and when I get a new release from the contributor, install my changes to the new release (of course, I will need to shake down the conflicts). Is there a open source product that works well on this kind of thing? Where can I find out more about it? thanks in advance, Warren Vail warren@vailtech.net