I've taken another look at version control systems again today. Subversion still seems the clear winner, despite Linus Torvald's writing his own called Git. It's a distributed SCM and designed for the Linux dev model, ie maintainers accepting patches, instead of the centralised SVN model where devs commit independently.
Word is that a distributed system is better for massively branched systems, but I don't quite see it. The killer for Git is that it's optimised for POSIX; Windows performance is poor.
Distributed runners up include Darcs and Mercurial. The former can become unstable, the latter I didn't read about any problems before I lost interest in the distributed class.
edit: huge fan of Mercurial now. Note all (or at least, many) of the Svn devs are now on Mercurial.