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.
*: 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.