CVSEdit
CVS or the Concurrent Versions System is a centralized, free version control system that has largely been replaced by Subversion. Old projects and existing infrastructure may continue to employ CVS but most new projects are built using a distributed version control system such as Git. Many existing projects with long histories stored in centralized repos are also switching to DVCS systems.
Those looking for a free, centralized (client/server) version control system choose Subversion. The principal commercial alternative is Perforce.