Google used this internally to manage questions at their all-hands meetings, letting the audience upvote good questions and avoid unimportant questions. Are there any other good solutions?

It's a real problem at conferences: many of the "questions" asked during the Q&A at public meetings are a waste of time, a chance to show off [1].

I found an example of Yik Yak used for this purpose [2]

[1] http://vooza.com/videos/conference-q-and-a/

[2] http://www.theconglomerate.org/2015/03/i-have-seen-the-futur...