Which Survey
There aren't many companies that I have found that I trust completely. The company that I have been most impressed with is:
YouGov are one of the leading Market Research companies in the UK, if not the world and their findings are published in various media all around the globe.
As a member of the YouGov panel you will be invited from time-to-time to participate in surveys on various topics. Each time you take part in a survey, you will have your account credited with cash amounts depending on the length and complexity of the survey, or you may be entered into a prize draw. When you reach £50, YouGov will send you a cheque for that amount.
YouGov also in not just for UK users. They acccomodate for users from the USA and from the Middle East region. Once you are on the website, you will find the relevant links to access these country specific areas.
My experiences with YouGov have only ever been positive and the same applies to others that I have spoke to regarding taking surveys online. They do not spam you with advertising, and they always pay you on time and when you reach the £50 mark.