What are some GUI clients available for Mercurial?

Tortoise HG. All the tortoise goodness, now for Mercurial.

UPDATE july 2020: The original official website linked above is abandoned.

The project moved to: https://foss.heptapod.net/mercurial/tortoisehg/thg

IMO best GUI Hg client for OSX is SoureTree - http://sourcetreeapp.com

I was using MacHg, which is ok (and free), but SourceTree has better support, ongoing development and better workflow.