I work on billing project with other 4 guys and we use CVS. It's very good free tool for different projects: small and big.

Also, try to look at subversion, because it will replace CVS. I've never tried to use this tool. I'm just planning to do it.

And there are two power utilities: diff and patch. But it's difficalt to use them for controlling of version in the big projects.

Update: Also, try to look at SourceForge Enterprise Edition. Many developers use it on SourceForge.

      
--------------------------------
SV* sv_bless(SV* sv, HV* stash);