BitBucket Mercurial Repository Hosting
https://bitbucket.org/
BitBucket is a free Mercurial repository hosting service that allows both private and public repositories. I guess at some point they launched unlimited private repositories, which is interesting. I'm not sure what they charge for now.
Github has always led BitBucket in features (I don't know how they compare now), but that's only for Git version control, not Mercurial.