I've created this wiki to keep track of useful HTML5 resources/references to complement my video+booklet tutorial: HTML5 NOW - A Step-by-Step Video Tutorial For Getting Started Today
Available for pre-order: http://tantek.com/isbn/0321719913
Updates that have been made to the HTML5 specification since HTML5 Now was published.
Sites with major conversions to HTML5 and testimonials
No browser "fully" supports HTML5 yet. However, web developers have written scripts that either detect HTML5 support, or emulate many features of HTML5.
Be sure that any time you use javascript to enhance presentation or make some CSS work, that your pages still work well with javascript turned off.
Sites that collect HTML5 examples
Sites that test and/or show HTML5 support across browsers - varying quality and datedness. Be sure to double-check the tests for yourself to make sure they're valid.