Mensa is an international society founded in Britain in 1946. The sole criterion for membership is scoring in the top 2% on any of several standardized intelligence or achievement tests.
International Mensa: In addition to a wealth of information about Mensa, this is a very nice Web site.
Tallahassee Mensa: Decent site maintained in Florida's capitol.
American Mensa: Many useful links for Mensans. An optically busy site.
British Mensa: Challenge yourself with their puzzles. The British do not endorse every new fad, like HTML 2.0.
Swiss Mensa: Nice little site. I belong to their team in the RC5-64 decryption competition.
On-Line IQ Tests: Darryl Miyaguchi's Collection of IQ Tests.
distributed net: Become part of the world's largest computer and break codes for charity.