Prolific links you academic researchers looking for people to answer questions to help aid them in their research and studies. In return you will be financially rewarded. Rewards vary from study to study, some from as little as just a few pence for a very short study, but can be several pounds for longer studies. Once you have earned £5 you can then cash out to a PayPal account. 

This tends to offer more frequent studies, but during university holidays, it can get very quiet!