iClickers and REEF - supported by TAMU Instructional Technology Services. Other than using the podium for projecting, iClickers were the first real technology I used in the classroom. There is a pretty steep learning curve and a few moments when you heart goes in your mouth... but overall entirely worthwhile trying. Dr. Cynthia Werner and I gave a presentation several years ago summarizing our first experience with clickers --- the link to the PDF of our presentation is here: Our First Experience with Clickers The new incarnation i-Clickers called REEF is not a product that I would use (it it's current configration)... it lacks in a lot of the basic functionality of other options (like Slid.do and PollEverywhere).
NearPod.com "an interactive classroom tool for teachers to engage students with interactive lessons." My students really liked this; however I found it a little cumbersome working on one screen (instructor) and projecting another (student) in the classroom --- again, it's a learning curve. It was really easy to upload PowerPoints to the NearPod website and integrate polls and questions.
bubbl.us "Brainstorm and easily create colorful mind maps to print or share with others."