Mensa is an international organisation of some 20,000 people, all of whom have an IQ that's higher than the average. Have you ever wondered whether you would qualify for Mensa membership? Although it's not an official IQ test that you can use to gain entry into Mensa, the organisation has a free "mental workout" on its web site that you can try, in order to guage your own skills.
There are 30 questions, and you are allowed half an hour to take the test. Once you've done so, you'll be graded immediately so you can find out how well you've done.
See www.mensa.org/workout to give it a try.