NGLayout Tier 1 CSS 1 Test Suite - A comprehensive (hundreds) set of tests that target individual style properties in CSS1. A professional quality "Tier 1" suite.
Official W3C CSS 1 Test Suite - Frequently updated with more complex tests than those in the Tier 1 suite.
David Baron's CSS Tests - David files more NGLayout CSS bugs than just about anyone (last time we counted). This document is a great example of a wonderful test case helper, with links directly to bug reports, etc.
Braden McDaniel CSS Tests - including some beautiful box model tests
W3C CSS 1 "Float" Tests - Some challenging tests

Please don't forget to file bugs if you find them!