Rendertests - CSS pages that can used to test with different browsers. - http://realdev1.realise.com/rossa/rendertest/
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. - http://centricle.com/ref/css/filters/
David Baron's CSS tests - Conformance tests covering many parts of the CSS2 specification. - http://dbaron.org/css/test/results
CSS Test Matrix - Features tests on various properties according to the CSS1 specification. - http://www.mozilla.org/newlayout/testcases/css/tier1.html
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. - http://www.w3.org/Style/CSS/Test/