I was vaguely aware of the Harry Potter Alliance, and I knew about things like the Project for Awesome, before today. In fact, I've long been enamored with the power of fandoms to bring people together to do good in the world. But I didn't know before seeing this video (about the Aurora, Colorado shooting) that there was a name for it: Fan Activism. It's nice to have something to google.
I also learned about Racebending, an organization that grew out of the Avatar fandom in response to M. Knight Shyamalan casting white actors for many of the principal characters in the series, which featured no white characters. They now more broadly advocate for the inclusion of underrepresented groups in mainstream media.
The best source I've found on the topic is Allison Jones's blog, in particular this post.