Overview of source control tools:
|System||Project page (external)||Market share (*)|
|SVN / Subverison||Apache subversion||55%|
|GIT||Official GIT site||7%|
|VSS / Visual Source-safe||Microsoft site||2%|
|TFS / Team foundation server||Microsoft site||2%|
One can also filter the different tools based on the OS where they are supported.
Of interest can be the post or this post with the statistics on source control tool usage.
*: Market share is taken from the different references listed above. It is not entirely accurate but gives a trend. You could also count the number of installations, users, servers, repositories, projects, … and have completely different results.