Mercurial, also known as hg, is yet another distributed version control system. It’s written in Python and includes “mercurial.el” using VC.
vc-hg.el is part of Emacs since version 22.2 (released March 2008).
DistributedVersionControl supports Mercurial.
Another Mercurial mode can be found at http://hg.intevation.org/mercurial/file/tip/contrib/mercurial.el. It’s being developed under XEmacs.
There’s another Mercurial mode, aHg, at https://es.fbk.eu/people/griggio/ahg.html .
If you like using Magit for Git, you will find it interesting to work with Monky - Magit for Hg.
Small article about using Emacs with mercurial.el