How to get a sweet job before you graduate
excid3.com
excid3.com
You already know that, but just having a github got people interested in me! Showing that I actually care about what I do and wanting to learn more and more.
Giving back is not only a bonus, it's a great incentive in itself that, like you said, you know you've made a difference in someone's life!
And, as for actually promoting yourself, it gets you to meet and communicate with awesome people! That's how I met you in the first place thanks to some rails problem I was having, got to talk with some cool people on twitter and even on IRC.