The mr(1) command can checkout, update, or perform other actions on a set of repositories as if they were one combined respository. It supports any combination of subversion, git, cvs, mecurial, bzr and darcs repositories, and support for other revision control systems can easily be added.

Comparision between git mercurial and bazaar

Illustrated introduction to distributed VCS

Project managment portal, with versionning tools (bzr, hg, svn, git) and trac