The comprehensive list of Internet Explorer's CSS support, broken down by version number. The links to definition for each value is extremely handy.
ie6
CSS3 PIE
CSS3 PIE (Progressive Internet Explorer) is an IE behavior that extends it's capabilities by providing support for several CSS3 features such as border-radius, box-shadow, border-image, multiple background images and linear gradients for background images.
Selectivizr
A "JavaScript utility that emulates CSS3 pseudo-classes and attribute selectors in Internet Explorer 6-8."