Subversion

And why, really? There exists a similar source control system, CVS. It is also free. It has similar functions. It is the most commonly used. To our opinion, the answer is not connected to the current state, it is in the future. SVN was really started from nothing, it hasn't solved all CVS problems yet, but from the point of view of internal architecture it is much better. This guarantees its rapid development, and in the nearest future SVN might become the best system for code versions control.

In the present SVN has a number of advantages (and disadvantages) over CVS, which can be reasons for choosing SVN as a source control system. You can view a detailed list of basic differences. Some people think that some of the differences put SVN into another class with respect to CVS. Examples of such differences are atomic commits and support of files and folders renaming. Though in our company we still use CVS 1.11p1 (a very old version) and need nothing better.

Thus, the choice is yours.