We use Git. It's been great so far but it is a bit complex if you're not used to DSCM. The amount of commands that you really use in day to day development is very, very small however. I haven't tried Mercurial myself but a friend of mine uses it. There's also monotone to try out. For those researching SCM tools and want to move away from the horrible Subversion, start with these links:
http://en.wikipedia.org/wiki/Monotone_%28software%29 http://en.wikipedia.org/wiki/Mercurial_%28software%29 http://en.wikipedia.org/wiki/Git_%28software%29