Mercurial -- a distributed SCM

Mercurial is a fast, lightweight source control management system designed
for efficient handling of very large distributed projects.

Quick start: http://www.selenic.com/mercurial/wiki/index.cgi/QuickStart
Tutorial: http://www.selenic.com/mercurial/wiki/index.cgi/Tutorial
Extensions: http://www.selenic.com/mercurial/wiki/index.cgi/CategoryExtension