GitHub is a very popular repository hosting for open source ruby projects. Git is a version control system similar to (but a lot better than) SVN. I use git hosted on GitHub for my current project, MakePlans - a booking/reservation system. But for many is GitHub not a viable alternative since it is not free for non open-source projects and while it is great for many to outsource backups and repository management, it is not an option for those who needs to host it in-house due to security or other internal guidelines.
Johan Sørensen from Shortcut, a ruby on rails company here in Oslo, made this amazing free open-source "GitHub-clone" (update: Gitorious was launched the same week as GitHub.). It has been available as a open-source project for awhile but recently Shortcut made a free public site available for those who do not want to install Gitorious themselves.
Gitorious boost most of the features that GitHub has:
On a day to day basis the source tree browser is what I most often use with GitHub. It is ajax-based and loads commit messages after the rest of the page is loaded. This is often a slow process in GitHub and something that works much better in Gitorious where no ajax is used.
There is not much I miss from GitHub, but I cannot see a way to constantly watch others repositories as is possible in GitHub. I follow various rails-projects in GitHub but I use it mainly as a form of bookmark so it is not a very important feature for me.
QT announced today that they will use Gitorious as their public repository for their cross-platform UI framework. Hopefully more will follow so Gitorious can gain the attention it deserves compared to GitHub.