As the oldest and largest high IQ society test in the world, Mensa International offers a variety of tests known as Mensa tests to score both intelligence and IQ. Mensa is composed of national groups and is centered in England. Membership to Mensa requires a score about the 98% on certain IQ or intelligence tests.
You can take many of the Mensa tests online, though none of them will qualify you for membership due to their unsupervised nature. Mensa International's website provides the Mensa Workout, a test for entertainment purposes only. It is not an IQ test, and the score will not qualify you for Mensa. However, the test is still fun to take and requires a graphical browser. The test will give you 30 minutes to answer 30 questions. You can also find your country's local Mensa website on the Mensa International homepage. Each country's website allows you to take the Mensa home test, similar to the Mensa Workout test. It will not qualify you for Mensa membership, but it does show a score that correlates to your IQ range and indicates the likelihood of success should you decide to take the Mensa. The Mensa Home Test costs $18.
If you're interested in becoming a member of Mensa, you'll have to take a supervised admission test such as the Stanford-Binet Intelligence Scales or the Cattell. You can also take the Mensa Application Exam, proctored by Mensa. The test takes two hours and includes two tests involving logic and deductive reasoning. If you score at or above 98%, you'll be invited to join Mensa. There is a $40 testing fee for most countries.