CSS - Contents and compatibility An excellent CSS compatibility chart, in which Internet Explorer version 5-8 appear as a striking red column in a field of green.
CSS current work & how to participate The official site for development of the CSS specifications. Provides an overview of all the modules, with links to the draft specifications.
CSS3 Gradient Generator A great online tool for exploring what you can do with CSS3 gradients.
CSS3 Generator A very handy online code generator that delivers CSS code to your specs, making it easy to get the syntax right and add vendor-specific variants.
CSS3 Border Radius Generator Easy way to begin playing with border-radius. You enter the radius for each corner, and it shows a preview and gives you the code with all the browser variants.