Obviously the library authors make this happen and I mean no slight to them. They rock. But this thread is about Github so I focus my comments on how they add value.

Github made it easier to share code and created a community. Now more people share. Github standardized project formats (which dougws points out below). It's easier to figure out what's in a library.

This makes it easier for me to build complex apps. It saves my company gob loads of time & money. I fucking love Github and I fucking love people who use it to share high quality code that I don't have to recreate.