I've just posted new poll on Software Configuration Management (SCM) tools. I'm interested to see how Subversion compares these days to CVS. And who is using commercial tools like ClearCase, StarTeam and Perforce? Check it out software-configuration-management here.

I use subversion. It is very easy to use and it has a nice plug-in to eclipse. Additionally, this seems to be the qyasi standard for java developers.

I prefer darcs, but must admit that as a standalone consultant I still meet cvs in the financial business. I have also used clearcase extensively. The configuration support for clearcase is excellent.
CVS is outdated, even as a pure version control tool.

What we really should go for, is something that do the version control, but also helps out on configuration control.
this is where darcs come into the picture.
GIT is another one, but darcs I prefer.