A free iphone app called Field Agent allows users to earn money by helping companies complete small market research tasks.
Field Agent app review
How does it work? Smartphone users who have downloaded the app can help provide provide 'immediate field intelligence and consumer insight by utilising technologies built into the iPhone'. Or in other words complete simple market research tasks. A company may want users to check prices in a certain store, take product photos, answer surveys etc. If there is such a task the app designers broadcast it to the app users and if they choose to accept and complete the task they will get paid.
How much can you earn?
Usually users earn between £3 and £6 a task. Money is paid to a Field Agent account which can be withdrawn via Paypal. While you won't get rich doing this you might earn some spare cash if you save it up over time.