Hubsyncer is a project mainly written in Python, it's free.
Uses hg-git (assumed to be configured for the server process's user) to sync a github repo to a mercurial repo via a post-receive hook.
A simple micro-app that accepts post-receive POST requests from github (see http://help.github.com/post-receive-hooks/) and then mirrors the specified repository to a mercurial repository elsewhere using hg-git.