A popular DistributedVersionControl system similar to GitVersionControl and BazaarVersionControl. Has historically had better support for MicrosoftWindows than those.
Was developed for the same reason as Git, to replace BitKeeper for LinuxKernel development, but Git was eventually chosen. Used in the development of TheMozillaProject, PythonLanguage (Mercurial is written in Python), VimTextEditor, GoLanguage, XEmacs and NetBeans.
Its official web page is at http://mercurial.selenic.com/