iclicker go: www1.iclicker.com
This website provides a brief video on how students can use their iphones or a tablets as an iclicker. The advantage for students is that they would not be required to purchase an iclicker, and would only be required to pay the fee for the software; which is pretty cheap: 4yrs for only $31.99 or on the other end, $9.99 for 6months (and a free 60 day trial)...not bad.
Those who have never used Student Response Systems in the classroom, this website will also fill you in on how to use this particular Student Response System (iclicker) in the classroom and its benefits. However, regardless of if you choose this Student Response System or another, the information is still very useful.