Hgsubversion is an extension for Mercurial that allows using Mercurial as a Subversion client.

At this point, hgsubversion is usable by users reasonably familiar with Mercurial as a VCS. It's not recommended to dive into hgsubversion as an introduction to Mercurial, since hgsubversion "bends the rules" a little and violates some of the typical assumptions of early Mercurial users.