Perforce (<http://www.perforce.com/>) is a commercial Version Control System (VCS) used by the FreeBSD Project to manage several trees that would be too difficult to hold in the main CVS trees for several reasons (including CVS shortcomings).
It is a small, centralised, fast and easy-to-use system but it lacks some of the features (such as offline work) that distributed VCS offer now. It also requires network connectivity for most of the time for practical and comfortable work (because it uses remote file "open" locks before files can be locally modified).
Perforce is, among many other things, used for Google Summer of Code projects.