Something nice and useful i spotted today. Peter Freitag has launched a documentation shortcut site that has indexed the W3C CSS 2.1 Specification. The usability is increased by a script.aculo.us based auto completion feature.
Additionally there is a Plug-in for Firefox that will add cssdocs.org to your search engine favorites, making this a very handy and fast to use tool, when you are looking for attributes of a specific CSS element.
So if you are a web developer be sure to bookmark http://www.cssdocs.org and if you happen to use Firefox as your preferred developing browser get a hold of the Plug-in (from the page take the link at the top, reading "CSS Documentation") aswell.