These are repos for various Mercurial development:
http://selenic.com/repo/hg - the main repository
http://hg.intevation.org/mercurial-tah - Thomas Arendsen Hein's outgoing changes
http://www.serpentine.com/hg/ - Bryan O'Sullivan's outgoing changes
http://hg.serpentine.com/mercurial/mq - Chris Mason's Mercurial Queues extension