Subversion or CVS are nice.

CVS are well-spread and is really concurrent. You normally don't lock the files but allow multiple persons to edit the same file. 19 times out of 20, CVS will correctly patch together changes from multiple users. When it sees that the changes collides, it will create a temporary file with your original data, and then patch the work file to contain both alternatives and let you manually solve.

The only disadvantage with CVS is that it doesn't version-handle meta-data. That makes it harder to rename files or move them between directories.

Supversion is basically a "next-generation" CVS and adds version handling of meta-data too.

Both the above choices have a good set of tools (command-line or graphical) for both Unix and Win32 users and quite a lot of programmers editors ir IDE allows integration.

One of the bad things with SourceSafe is that it compresses the file data. Todays harddisks are so large that you don't gain much by that. But a binary file means that if the source code repository gets broken, you may continue for days or weaks without knowing that you can't restore older versions. And when your backup has overrrun, you can't manually extract data from the binary file.