HTML5 Readiness : Visual timeline of HTML5 feature support.
When Can I Use… : Up to date HTML5 feature browser support reference.
HTML5 Test : Series of browser tests to evaluate feature support.
HTML5 Infographic : A quick overview of scope and support.
W3C HTML5 Web Author View : Lightweight version of the W3c spec, perfect for web developers.
Chrome Implementation Status : Follow the engineering effort for cutting edge support from the Chrome team.
Dive into HTML5 by Mark Pilgrim : Learn HTML5 and have a laugh at the same time.
HTML5: Comparison of Layout Engines : If the Wikipedia says it then it must be true.
Safari HTML5 Guides : Excellent WebKit-oriented development documentation.
Mozilla Development Center HTML5 Reference : Excellent documentation and guides.
WhatWG Web Apps Spec (HTML5 plus the kitchen sink) : Working draft spec of everything under the larger HTML5 umbrella.
ECMAScript 5 table : Detailed table of ES5 support across browsers.
Compatibility Tables : Results of DOM and CSS properties tests in major browsers.

Modernizr : Javascript library for feature detection and control fallback cases of HTML5.
html5 shiv : Javascript patch to make IE support, and print, the new tag elements.
CSS Button Maker : Preview CSS buttons with a fancy interface to play with the parameters.
CSS3 Generator : Preview CSS buttons with a fancy interface to play with the parameters.
CSS3 Sandbox : Test and get the code of the fanciest CSS features.
CSS3 Gradient Generator : Generate CSS code for gradients on the fly.
CSS3 Please : Use CSS3 without having to worry about xbrowser differences.
Font Dragr : Test your fonts using drag and drop.
Font Squirrel Generator : Most complete tool to generate code for you font-face support.
Let’s make the web faster : Complete list of tools related to web performance.

HTML5 Watch
CSS3 Watch
HTML5 Doctor
WhatWG Blog
HTML5 Demos
Chromium HTML5 Discussion Group
Mozilla Hacks
Safari HTML5 Demos
Test Drive IE9 Demos
Open Web – Opera Developer Community
Chrome Experiments