A very handy tool to generate the rather complex CSS required to generate gradients. This is by far the simplest method that I've seen so far.
“20 Things I Learned About Browsers and the Web” is a short guide for anyone who’s curious about the basics of browsers and the web." The illustrations are great and the implementation (using the canvas tag) is slick.
This is the best breakdown of selectors I've seen. An excellent reference for all Web developers.
The comprehensive list of Internet Explorer's CSS support, broken down by version number. The links to definition for each value is extremely handy.
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.