Alan Gutierrez compares Sourceforge and Github. In short: Sourceforge is for projects, while Github is a place to put your code. It doesn’t have to be a full project. It doesn’t have to work. It’s just a way to share your code with other hackers who might be interested. And they can fork it and start collaborating.
The culture of traditional open source software projects (like the Apache, MySQL, Subversion, or Gnome) treats forks as a negative and disruptive force. When XFree86 was forked into Xorg, for example, there was much drama in the community. Or note Josh Matthews’ hand-wringing over free software's hidden burden.
With Git (and other decentralized revision control systems), forking is a normal and healthy activity, and a part of your everyday workflow. Fork and merge all over the place! Unfettered experimentation! This is next step in the evolution of open source.