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TortoiseHg is a GUI front-end for Mercurial that runs on Linux and Microsoft Windows, on which integrates directly with File Explorer. 
It is written in PyQt, and the underlying client can be used on the command line.
This is a brief list of its features:
- Repository explorer
- Commit dialog
- Support for visual diff/merge tools.
- Data mining on repository contents
- Seamless support for serving a repository via Mercurial's integrated web interface.
- Repository synchronization
- Intuitive GUI for managing Mercurial settings
It is free software released under the GNU General Public License.