An article in the Independent recommends Opinionpanel amongst others as a site for students to earn a little extra money.
About Opinionpanel, now known as YouthSight, the article says:
Any opportunity to earn a little extra money will be greatly appreciated in studentville, so this site should be added to your favourites. It's a market research company that focuses on students, and they pay you for answering certain surveys and questionnaires. Although, the money comes in the form of Amazon vouchers rather than sterling, it all adds up. The questions are easy, there's no spamming, and your responses could even help to shape public opinion about student life.