The Concurrent Version System (CVS) was selected for use by the OpenBSD project group. With CVS, seventy or so developers are able to update the source code (which is around 1.2GB, around 140,000 files) that comprise the OS, ports/packages, and the X Window System. A large number of OpenBSD users then download the OS sources via anonymous CVS.
CVS also allows easy creation and maintenance of branches so that we can provide a release + patches source tree and an up to the minute development source tree with parallel development. With CVSweb, a convenient and navigable web accessible interface to all of this is available as well.
Since CVS interfaces with the Secure Shell (ssh(1)) it can provide reliable user and host authentication. This fits in well with the OpenBSD project's goal of security.