This website is different from the other products reviewed here in that it's geared towards testing rather than improving your brainpower. However, a bit of time spent putting your mind through its paces should reveal your strengths and weaknesses, helping you decide where you need to put in that bit of extra effort.
The site is a selection of links to tests on third-party sites. It's far better maintained than most link-intensive sites, though, as we didn't unearth a single broken link. As well as IQ tests, there are also personality, emotional intelligence, aptitude and entrepreneurship tests.
