The Acid tests are a test to check teh compatibility of a web browser with some standardized tests. Acid2 was the standard for so long but Acid3 test was recently released and no browser passed.
With Acid2 there were several browsers that passed.
Safari for both Windows and OS X
Opera
Firefox 3 beta
IE8
But now with Acid3 no browser passed.
More info:
The Web Standards Project