Ever since its inception, Geeklog has been using CVS as the source control system. We have discussed moving to a more "modern" system (e.g. Subversion) on more than one occasion, but the benefits always seemed minimal. And so we kept on using CVS.
Fast-forward a few years and a new class of version control systems has arrived: Distributed version control (DVCS). These systems come with support for easy branching and merging built in, which allows for parallel and experimental development that can still be merged back easily into the main development line. And a checkout is a self-contained repository that can even be used offline (and changes can be merged back later).