Get lots of opinions in a short, visual way
PollEverywhere is a great tool for things like anticipatory set, establishing prior knowledge, summarizing, or surveying an audience. My middle school kids love it because they can use technology to participate, essentially in the same way you would with a clicker response system, but using technology that already exists in the classroom (cellphones, computers, etc.) I loved it because I got a quick idea of a broad array of opinions. Additionally, it's a really great method to get all students participating, even the ones who may be too shy to speak up otherwise. Kids often assume that everyone thinks like them OR that they're the only one that thinks something. This method allows them to see where they stand in comparison to others in reality, without actually raising their hand in class. While there are many other tools that allow for this (like Twitter, etc.), this allows them to do so without the headaches of creating usernames, passwords, etc. I think it could definitely serve as a teaching tool to help kids consider the opinions of others as valid points of view, and working as a class to model that it's possible to disagree with an opinion in a rational manner (something my students need lots of practice with!)
One issue I ran into was making sure kids put the right code in, when submitting their answer (for poll or multiple choice questions), but the short answers work great! The only other issue I have occasionally run in to is kids submitting inappropriate answers, on short answer questions. Depending on one's purpose in using it, you could just make sure not to display the results on the screen, at least temporarily, or have certain kids that you know may be a problem submit their answer on paper to you, and you could enter it online.
How I Use It
I've used this tool for things like seeing what kids already know about a topic, having them summarize what they learned after a lesson, and getting their opinions on various topics. Since I teach technology, I've used polls asking about their favorite form of social media (they love seeing the bars move as people answer), and their level of understanding of a particular topic (using keywords to search online), before and after a lesson. It could also be used as an exit ticket, demonstrating knowledge of a topic, as long as they put their name with their answer. My favorite extra feature is that you can turn the answer results into a word cloud on Wordle with just a few clicks. My kids were super impressed with that part, as was I :-) I had them submit words they thought of when they thought of technology during our first class of the year. Not surprisingly, the biggest answer on the Wordle was Facebook.
I have also used this extensively for professional development sessions with teachers. For instance, I run Edcamp Phoenix, and I used it as part of our opening session, asking a poll about who had heard of Edcamp before, and an open ended question about what they knew about it. At our closing session, we used it to find out things that teachers learned during the day and ways they imagined the model could be implemented in other settings, both open-ended questions. It really is a great way to have everyone share their thoughts, and see others' thoughts in a short amount of time, particularly in large groups.