If you're fit and healthy – and prepared to accept the risk of tests - you could earn up to £150 a day by taking part in medical trials. Website gpgp.net offers a whole load of information on what to expect (go to the UK section), and where to apply. Note though that it's sponsored by BioTrax; one of the biggest medical trial recruiters in the world.
How much? Depending on what it is you’re being tested for, you can earn between £70 and £150 per day.