Subversion
The goal of the [http://subversion.tigris.org Subversion] project is to build a ["CentralisedSCM"] system that is a compelling replacement for ["CVS"] in the open source community.
If you're using Mercurial but need to collaborate with people using Subversion, see ["SubversionToMercurialSync"] and ["SubversionToMercurialHowto"].