There are several online sites that allow for polling of students in an informal manner. All have a free version with some limitations and the availability to purchase more options
Poll Everywhere - http://www.polleverywhere.com/ - Allows for polling through text messages, Twitter, and/or a website. The teacher signs up for an account, creates a poll question, gives the students the text number or website, and students answer with their devices. The questions can be embedded into PowerPoint or Keynote and results are viewed live there or through the website.
The Free version allows for only 40 responses per poll, but multiple polls can be create and cleared out before the next class period.
Mentimeter does basically the same thing as Poll Everywhere. Once again the teacher will sign up, create their question, give the student the website or text number, and watch real time results. I believe this one can only view results through the website. Mentimeter does have a few more themes to jazz up your poll. Check out their How-To.
Polldaddy - http://polldaddy.com/ Another program similar to Poll Everywhere. Their Free account is for one user and allows for unlimited polls, surveys, quizzes, responses, and votes.There is also a Polldaddy app for iOS, but it looks like it is more for multiple people answering questions on one iPad instead of an entire class worth of responses.
TodaysMeet - https://todaysmeet.com/ Thinking a little outside the box here, but TodaysMeet could be used in a 1:1 or BYOD for some formative assessments. TodaysMeet is especially good at backchannel conversations. A teacher could pose a question and allow students to all answer in TodaysMeet. There would then be a record of all student responses.
Teachers go to the site, name their room, and set the room to close (keeps responses one hour to one year). Then click the create your room button. Give students the URL and ask your question. Students will type in their name and join the room and answer the question.
Many people will mention Twitter in this conversation as well. While I believe Twitter has many great qualities, a few minuses; Twitter is public and viewable by everyone, not all students will have accounts or be allowed to have accounts, and it would be very easy for something inappropriate to get through from the outside world into your class discussion. So while I think it is great for teachers to use for the PLC/PLNs, great for twitter chats and professional development, and great for teachers to post educational information about their classrooms, I am not sure I am ready to add it to this list for polling and formative assessments.