I suppose any good VCS should work fine. Each dev should get his/her own branch which is periodically merged upstream. We use SVN and it does the job just fine

I suppose any good VCS should work fine. Each dev should get his/her own branch which is periodically merged upstream. We use SVN and it does the job just fine. In fact, we are still stuck on 1.4 so we track merges ourselves which isn't really a big deal.