If you worked on software in collaboration with multiple developers, then you've probably used (or wished you used) some sort of revision control system. The Google Search Volume Index plot below suggests some trends surrounding the currently available tools.
(Note: by no means is this very scientific, due to the fact that people searching with these terms could have been searching for something entirely different.)
CVS, although huge in its time, is on the decline, while SVN, Git, and Mercurial are on the rise. I have used plenty of CVS and SVN to be ready for change. I am now using Git which I have really liked so far. If you have already been using SVN as I had, I would recommend the Git-SVN Crash course to get started quickly.