GitHub is a project hosting service for projects using the Git revision control system. It is written in Ruby on Rails, being developed by Logical Awesome (Chris Wanstrath, PJ Hyett, and Tom Preston-Werner).
GitHub operates a pastebin-style site at gist.github.com
Used by
Microsoft has created a repository for IronRuby projects, to be hosted on GitHub.
Other notable projects that use GitHub as their hosted revision control platform include:

With Sprout, you can build, publish and manage widgets, mini-sites, banners, mashups and other rich media Web content in three easy steps:
1. Build – Sprouts can be of any size and any number of pages and can include images, video and audio and components such as slideshows and jukeboxes as well as Web service components such as Twitter, PollDaddy, ChipIn and more.