Introduction to HTML5 by Robert Nyman
HTML5 Demos and Examples
On html5demos.com by Remy Sharp you find some code examples, table with supported features of HTML5 in different browsers and some experiments with HTML5. The code is Creative Commons and can be forked on GitHub.
Dive into HTML5
“Dive Into HTML5 seeks to elaborate on a hand-picked Selection of features from the HTML5 specification and other fine Standards”
Dive into HTML5 is an online book by Mark Pilgrim where he goes into different bits and pieces of HTML5. He writes about stuff such as Detecting HTML5 features, Canvas Support and Offline Support. Recommended reading.
HTML5 Showcase by Apple
Apple has made some clear statements the last couple of years supporting HTML5 over proprietary software such as Flash from Adobe. Their top sellers iPhone and iPad does not support Flash and will rely on HTML5 for richer interfaces on web apps. They have set up a landing page where you can find HTML5 demos that shows what Safari can do.
Google has a website named HTML5Rocks where they push the new standards as well. You can find a studio with examples of what can be done with these standards, a code playground where you can easily start playing with the code behind all this fancy magic. They also offer tutorials and and interactive presentation on HTML5. Feel free to contribute code to the project to show off how HTML5 rocks.
When you just want to start building stuff that works in all major browsers there are some stuff that needs to be set up, a lot of boilerplate code, lucky enough Paul Irish and Divya Manian already did that for you. From time to time I use the HTML5 Boilerplate template for getting HTML5-frontends up quick. It is also available directly from GitHub.
Introduction to HTML5 by Mozilla
On MDN there is a list of links to different HTML5 articles covering different parts of the new standards and features that build up HTML5.
Chrome Experiments is a showcase for creative web experiments, the vast majority of which are built with the latest open technologies, including HTML5, Canvas, SVG, and WebGL. All of them were made and submitted by talented artists and programmers from around the world.
Internet Explorer Test Drive
With Internet Explorer 9 Microsoft finally delivers a browser that more or less supports the standards web developers and designers would like to use in order to utilize the web as it was intended. Microsoft has released a Internet Explorer 9 Developer Guide and a page for HTML5 Demos that show off the new browsers capabilities.
Acid3 Browser Test
Test how well the different browsers support a subset of the standards available for web developers and designers. Primarily Acid3 is for web browser vendors to ensure that their browsers match a set of features that enable a richer and better web.