I saw the Socrative student polling app demonstrated at the Music Ednet ‘Daytime‘ music conference.
Socrative is free (basic use) and consists of a teacher module where quizzes, questionnaires and resource materials are assembled and a class module where students logon (via the website or app) to take part in the poll.
Our demonstration had us answering questions about our music-technology abilities and our involvement in courses focusing on scoring for movies.
The app worked flawlessly, updating voting / scoring in real time.
Once home I tried out the teacher app (on iPad) and found it to be easy to use if perhaps a little bare-bones.
Socrative should prove useful in any curriculum area and could be used to gain student feedback, test basic concepts or administer assessment tasks.
The app provides a range of reports and the option for responses to remain anonymous. The help facilities are impressive, with a free pdf manual to make operation and quiz creation painless. Personally I liked the idea of the ‘Space Race’ the best; students are placed in teams which compete to race across the screen. Huge fun!
Socrative Website 
Socrative Manual 