Revision graph
This is now included in the development repositories, should be in the next release. There's [http://hg.xavamedia.nl/mercurial/crew/graph an example].
Older versions
An experimental hgweb feature to visualize [:Branch:branches]. There is no way to turn it on as it is not integrated into the mainline (yet).
attachment:brview-preview.png BR [:BranchView/shot1:screen shot]
http://selenic.com/pipermail/mercurial/2008-March/018113.html
http://selenic.com/pipermail/mercurial/2007-November/015782.html - contains link to [http://www.blacktrash.org/hg/hg-branchview/ example repo]
http://selenic.com/pipermail/mercurial/2007-November/015805.html - Matt's opinion