GitHub

From APIDesign

GitHub is the source control hosting provider that popularized Git. My feeling is that Mercurial and Git are on par from the end user point of view, but the enormous popularity of GitHub just shifts more developers towards Git. But why stop at a single repository? Let's connect multiple Git repositories together and orchestrate MultiGitRepository - that's the challenge.

Correction: Git is better in handling branches than Mercurial. That's what I can tell after initial five years with Mercurial and last three years spend mostly with Git.