CVS - Concurrent Version System
Homepage: http://www.nongnu.org/cvs/ License: Open Source
Concurrent Version System or CVS is the most popular source control system in the open source world, at this time. The subversion system is likely to overtake it. CVS was designed to add the ability for multiple developers in seperate locations to work on the same project
Nonetheless, it has popularized the CheckoutMergeCheckin? mode of source control. The oldest form is CheckoutLockCheckin?, popularized by systems like RCS and SCCS.