I've only used subversion http://subversion.apache.org/ and git http://git-scm.com/ , the later was a bit difficult to get used to since I had just become somewhat accustomed to svn, but once you get the hang of it it's pretty easy to use. I believe git is more widely used these days, I hear mercurial (hg) http://mercurial.selenic.com/ is the next big thing.