These are not exciting sites as such, but they contain a lot of current information about the main technologies we use as web developers, and I highly recommend readers learn what they cover and how to get to them when you need them.
Two of the sites are rather lengthly, detailed references for working with web development issues and because they are current information they are worth knowing about. “Web browser standards support” is a lengthy listing of support for standard web technologies by popular web browsers (IE6, Firefox 1.0, & Opera 8). The technologies include: HTML, CSS, DOM, & Miscellaneous technologies (Atom, RSS 1.0, RSS 2.0, SOAP 1.2, XFORMS 1.0, XHTML 1.0, 1.1, etc.)
Web browser Standards Support (http://nanobox.chipx86.com/browser_support.php#otherbrowsers).
Internet Explorer to Mozilla Migration Guide
And this chart covers all of CSS1 and CSS2 properties and tags with compatibility info that covers 20 different browser versions, and is from the CORECSS book series. Handy in that it allows you to select just one browser and see its complete compatability list by itself. Too bad it is not expanded to CSS2.1 and future CSS3.0 projected additions to CSS.
Full Css Property Compatibility Chart