These are mostly the sort of non-commercial/political surveys you see reported in the press (they also do a regular BrandIndex one but that goes on a bit).
Generally they are short and interesting - the last one I did was about TV (eg do you like Reality TV or not) and books (eg do you read a lot/a little/never; which books do the following characters appear in) - take about 5 mins and pay 50p or £1 a time. Occasionally you get a prize draw entry for eg £250 rather than the small payment, and there are also some to complete out of interest for no incentive at all.
Sign up here if you're interested!