Today it became more clear why did Phil Haack leave Microsoft to join github.com - GitHub for Windows has been released (https://github.com/blog/1127-github-for-windows). What’s GitHub? To answer that, I need to step back a little to tell you what Git is. Git (http://git-scm.com/) is most likely the most widespread known distributed source version control system in the world. It was created by famous Linus Torvalds (yes, THAT Linus T.) as a replacement to BitKeeper as Linus decided that the other tool doesn’t cover all his needs regarding support of Linux kernel development. Git got popular very, very quickly, especially in communities gathered around OSS (mainly Java and linux related).

What’s GitHub? It’s the internet repository of repositories, running on Git. For free (well, there’s a freemium model…) you can start a repository for your project, so your bunch of geographically distributed developers (or just everyone you want) can access it from wherever they want. Of course, there’s also a social aspect - you can track other projects (that allow people to have them tracked) or even make your own forks (if this particular license covers that scenario). Isn’t it a OSS programmer’s wetdream? Sure it is, that’s why GitHub is most likely the most popular site in that category. It can be found here: https://github.com/
From the Microsoft Tech Devs perspective, the main problem with Git is that it didn’t care much about Windows. Git binaries for Windows were unofficial and until today they are published as "Preview" (sic!). That’s why Git was not getting that popular in .NET communities (as Mercurial / Bazaar is). But now, this may change - at http://windows.github.com/ you can find a dedicated (wow, Metro!) native Windows application that aids you with access to your repositories. It’s very simple (so far), but it’s some kind of indication that GitHub (and Git in general) is starting to face .NET developers as well.
P.S. And what’s Octocat and what does it have in common with Git? You can check that here: http://octodex.github.com/faq.html