Revision graph
This is now included in the development repositories, should be in the next release. There's an example.
Older versions
An experimental hgweb feature to visualize branches. There is no way to turn it on as it is not integrated into the mainline (yet).
http://selenic.com/pipermail/mercurial/2008-March/018113.html
http://selenic.com/pipermail/mercurial/2007-November/015782.html - contains link to example repo
http://selenic.com/pipermail/mercurial/2007-November/015805.html - Matt's opinion