Darcs - Darcs is a free, open source, source code management system which uses a decentralized model for tracking changes. Here are some good tips on running darcs that you may find helpful as a developer.
CVS - CVS is a free, open source, souce code management system which uses a server-client model for tracking changes. Here is a free book (PDF) on using CVS, Open Source Development with CVS, 3rd Ed.