HTML5Rocks

HTML5Rocks is an online resource by Google that regularly shares various resources on HTML5 and CSS3

The site has very detailed tutorials that digg stuff like HTML5 video or @web-fonts which are good starting points on the subjects.

An online playground with shortcuts to JS APIs + HTML-CSS properties helps trying new codes and seeing the outputs instantly.

And there is HTML5 Studio, which is a showcase of HTML5 codes like page-flip, video player, carousel, geolocation and more that are open source. It is possible to get inspired from them or re-use them in our projects.