Revision graph
This is now included in version 1.1 and later. 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