I recently made my first contribution to an existing open source project, and it was an interesting experience. One of the things I always wondered was, how do I go from "Ok, I have a bug fix / new feature" to "Great, it's part of the software now!". For this particular project it was as simple as forking the Github repository, making my changes, and sending a pull request- done. I could leave that Github repo on my account to easily show off my contribution to friends or potential employers, or delete it if I felt it was clutter. I didn't have to learn how `git am` or `git send-email` work or making a patch and sending it to a development list or anything like that.
And if someone wants to contribute to a project of mine on Github, but they don't want to use Github's system, that's cool too. If you're willing to help me out, I'll gladly learn how to take your contribution and get it into my project.
I honestly believe Github is the best thing to happen to OSS in a long time.