Subversion Version Control System
Homepage: http://subversion.tigris.org/ License: Open Source
Subversion was originally designed to be a better CVS, so it has most of CVS's features. Generally, Subversion's interface to a particular feature is similar to CVS's, except where there's a compelling reason to do otherwise.
Subversion has since expanded beyond its original goal of replacing CVS, but its history influenced its feature and interface choices; Subversion today should still feel very familiar to CVS users.