Centricle - CSS Filters
Table indicating which browsers are affected by the various methods used to hide CSS. Includes test cases for each technique, as well as user contributed comments.
CSS Test Matrix
Features tests on various properties according to the CSS1 specification.
CSS-Discuss
Wiki companion to the CssDiscussList mailing list. Includes tips and tricks and information on browser hacks, text sizing, and testing.
David Baron's CSS tests
Conformance tests covering many parts of the CSS2 specification.
Official CSS1 Test Suite from the W3C
Provided as a way for vendors and page authors to test their browser's conformance to the CSS1 specification.
Rendertests
CSS pages that can used to test with different browsers.