There are lots of choices about what sort of revision control system to use. The key decision you need to make is between centralized and decentralized repositories. Centralized revision control systems provide a single repository which people can check out, and check back in to. Decentralized repositories allow for more complex workflows where people branch your codde into their own repository, and make them otherwise available to you and others as needed.
While there are lots of options for both flavors, [http://subversion.tigris.org/ Subversion] and [http://git-scm.com/ Git] are currently the gold standard for centralized and decentralized revision control respectively. The good news is that if you would rather not run your own repository [http://code.google.com Google Code] and [http://github.com/ GitHub] provide free, rock-solid svn and git repositories, respectively, for
anyone who wants one.