I came across http://mentimeter.com/ a couple of weeks ago from a facebook post. It is one of those sites that allows you to poll people during a session or class. Unlike a lot of similar sites it does not require any subscriptions, annual plans and nor does it cost the price of a text to vote. I have wanted to include voting in my talks for a while but for the all the aforementioned reasons it could never be effectively done. So simple it can work both for talks and in class (either online or face to face). Simply go to the site, write your question and you get a unique URL.