You must have fully read the main VersionControl page for a set of requirements and processes currently used by the FreeBSD Project in its day-to-day operations, from basic VCS usage to release engineering.
Hg is being used by several FOSS projects and vendors.
OpenSolaris has selected Hg as its main VCS
Xen has been using Hg for several months now
Audacious Media Player project switched to Hg recently
Some people are now converting entire repositories and exposes them to the world:
FreeBSD has several converted repositories available here