Recently I came across this excellent CSS reference website:

http://reference.sitepoint.com/css

I highly recommend this website since it has been written by two of the world’s most renowned CSS experts — Tommy Olssen and Paul O’Brien. In this online reference, the entire CSS language is clearly and concisely explained, including browser compatibility, working examples, and easy-to-read descriptions.

For the purists, you can refer to the W3C CSS reference here - http://www.w3.org/TR/REC-CSS2/

Few other decent references are:

• http://msdn2.microsoft.com/en-us/library/ms531209(VS.85).aspx
• http://www.westciv.com/style_master/academy/css_tutorial/
• http://xhtml.com/en/css/reference/