Where did you start? I'm gonna assume here (http://dev.w3.org/html5/spec-author-view/, http://diveintohtml5.org/); suggestions, examples, blogs, resources, insights, blah, blah are appreciated as always.
http://www.w3schools.com/html5/default.asp
http://simon.html5.org/dump/html5-canvas-cheat-sheet.html
http://diveintohtml5.org/peeks-pokes-and-pointers.html
Tutorials
http://www.html5rocks.com/tutorials/
https://developer.mozilla.org/en/Canvas_tutorial
Demos
http://lab.simurai.com/css/buttons/
Sandboxes
http://playground.html5rocks.com/
Tools
http://www.contrast.ie/blog/speeding-up-with-modernizr/
http://css-tricks.com/examples/ButtonMaker/
http://labs.thecssninja.com/font_dragr/
http://www.fontsquirrel.com/fontface/generator
http://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-brows...
Compatibility
http://www.quirksmode.org/compatibility.html
Detecting HTML5 Support
I also like http://jsfiddle.net/ as a sandbox.
* "JavaScript: The Good Parts" by Crockford
* "JavaScript: The Definitive Guide" by David Flanagan
* MDC's "A Re-introduction to JavaScript": https://developer.mozilla.org/en/A_re-introduction_to_JavaSc...
* Resig's "Learning Advanced JavaScript": http://ejohn.org/apps/learn/
* MDC: https://developer.mozilla.org/en-US/
* Crockford's website: http://javascript.crockford.com/
* YUI Theater (especially Crockford's JavaScript talks): http://developer.yahoo.com/yui/theater/
(Not certain that the quality of information in my screen cast is nearly on par with the other info presented here, but it was a crowning achievement and something I'm proud of. Let me know if this is bad form, I'll remove this comment.)
Also its worth checking http://ie.microsoft.com/testdrive/ MS have some interesting sandboxes you can play with there. Learn your way around.
The authors do a pretty thorough job of reviewing the semantics of new (or updated) HTML5 elements.
On the other hand, js rocks...