CVS's shortcomings have inspired many efforts to replace it, and in the last few years many of these projects have reached usability. CVS's days as the uncontested default choice are slowly coming to an end. Following are all the other open source version control systems I was aware of as of early 2005. The only one I use on a regular basis is Subversion, that's why it's listed first. I have no recommendations for or against any of the others; a lot of information here is taken from their web sites. See also http://en.wikipedia.org/wiki/List_of_revision_control_software.
Subversion was written first and foremost to be a replacement for CVS—that is, to approach version control in roughly the same way CVS does, but without the problems and feature omissions that most frequently annoy users of CVS. One of Subversion's goals is for people already accustomed to CVS to find the transition to Subversion relatively smooth. There is not space here to go into detail about Subversion's features; see its web site for more information. [Disclaimer: I am involved in Subversion development, and it is the only one of these systems that I use on a regular basis.]