Mercurial (HG)
Mercurial is a free, distributed source control management tool. It efficiently handles projects of any size and offers an easy and intuitive interface.
mercurial.selenic.com
Git
Git is a free & open source, distributed version control system, not dependent on network access or a central server.
git-scm.com
Subversion
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.tigris.org
CVS
"Concurrent Versions System" (Source code version control)
www.nongnu.org/cvs/
RCS
"Revision Control System" (Source code version control)
en.tldp.org/HOWTO/RCS-1.html
PRCS
"Project Revision Control System" (Source code version control)
prcs.sourceforge.net
Aegis
(Source code version control)
aegis.sourceforge.net
Microsoft SourceSafe
(Source code version control)
Using CVS for Web development
philip.greenspun.com/wtr/cvs