Because cell phones are nearly ubiquitous and text messaging is definitely mainstream (at least in the United States), providing quick polls that can be answered with text messaging makes sense. Most college students have cell phones, so these spontaneous polls can be created "on the fly" in the classroom for immediate feedback to the instructor.
Students do not always want to answer questions in front of their peers for fear of being embarrassed after a wrong answer or too much attention from the instructor. Text messaging lets students answer anonymously, still giving the instructor a sense for the understanding of his/her class.
A couple of years back, I got excited about Google Documents, and the surveys, quizzes, or polls that can be created with them. I had forgotten about them, recalling how clunky and unintuitive they were to use and create, but I have taken another look at them recently. They are free. With that in mind, it takes a few more steps to get some things accomplished. The results of a survey are listedin a spreadsheet format; however, the three tools listed above can automatically display results in visual graphs, which are much more appealing.
Still, a lot can be done with Google Documents, and I do not believe that users are limited to a certain number or responses received to polls/quizzes/surveys or number of surveys created. On the other hand, the three tools mentioned above do limit users to 100 responses a month, or 20 responses per survey, or 40 audience members per poll.
Below is a presentation I created for a workshop yesterday: