This test was discussed in Malcolm Gladwell's book
Blink and is available in the "Demonstration" section
here. The test reveals unconscious attitudes and biases which can be quite different from the stated beliefs of the test-taker.
Of the tests available, the most well-known is the Race IAT. One surprising result: half of the 50,000 African Americans who took this test have a stronger association with whites than with blacks. Gladwell observes, "we live in North America, where we are surrounded every day by cultural message linking white with good."