Surveys. They can be a bit boring to fill in, but you can get money for it.
I would advise you to read the guide to surveys over at Martin’s Money Saving Expert.
There are a lot of scamsters in this area so be careful. You might want to read up on how to avoid online survey scams, and always try to make sure any company you sign up with reputable. Before you sign up, do a quick Google search for the company name or website address. Many bad reports? Do your research and make a judgement.