Song People

This survey site is a bit different to your usual. You listen to music and therefore can influence what is played on radio stations and such like. Each survey involves listening to music and stating how much you like each track.
This site is in the process of updating. At the present time each time you complete a survey you either win a CD or entered into a competition for some great music related items. The new system involves earning points for each survey you complete and I guess picking a prize of your choice. (Don't quote me on that one)
They send regular survey invites so you have lots to choose from.

http://survey.songpeople.com