Do you need another version control system which is more scalable than Visual Source Safe? Are you a CVS user who finds the lack of directory versioning a difficulty? Are you paying too much for your current version control system? Do you not use a version control system right now?
If you answered yes to any of the above you need to look at the Subversion version control system.
The original design goal of Subversion was for it to be a compelling replacement for CVS. However, it is also an excellent version control system in its own right.
Subversion features include atomic commits, tracking renames, versioned meta-data, versioned directories, cheap branching and tagging and efficient handling of binary files. See Subversion Features for the full list.
There are many Subversion related projects developing GUI interfaces for many platforms, IDE plugins, and conversion tools from other version control systems. See Subversion Projects for a selection of these projects.
There's quite a list (see Subversion Features) but essentially it comes down to a couple of important things.
It is stable. Your data is safe, provided you set up your server as described in the documentation.
It behaves itself. CVS has a number of gotcha's. With CVS if you change the last changed date on a file CVS thinks that the file has changed content, Subversion keeps a local pristine copy of the files you checked out so it can compare the files and really know you have changed it. CVS doesn't track the history of directories which leads to various usability issues, Subversion stores a revisioned directory tree in it's repository so it can properly track and merge changes to directory content. Visual Source Safe doesn't cope well with large repositories, Subversion does.
You might be thinking of starting to use a version control system for the first time or maybe considering changing to a different version control system. If you aren't currently using Visual Source Safe then the chances are that someone in your organisation is suggesting it because it's from Microsoft and the price is right. You need to read the Why Not VSS page.