Getting started with HTML5
We can argue about the merits of this terminology, but that ends up being like arguing about whether the 21st century began in 2000 or 2001 or whether AJAX is a useful term. The purists may be technically correct, but I'm going with standard usage by the majority.
Basic Cultural Re-education
The best place to start, especially for info on HTML5 as HTML (not all of the other specs under the HTML5 umbrella) is Mark Pilgrim's Dive into HTML5. This is a book in progress, developing along with the spec, but is very readable and the finished chapters capture not only what is new about HTML5, but why it matters. Mark also put HTML5 technologies, such as downloadable fonts, to good use in the online book.
For further tutorials and an interactive playground, HTML5 Rocks looks like a good resource, although I haven't fully explored it yet.
For further detail and ongoing Q&A, the HTML5 Doctor blog is a great resource. They explore what you can use from HTML5 in your work today, how to use semantic elements correctly, how to work around current browser limitations, and they answer questions from readers.
For tracking the latest implementation news of the CSS3 family of specs, the CSS3.info blog is great. That site also has preview pages to demo relevant CSS3 in use, as browsers implement it, and tracks both status of the specs through their relevant working groups, and also browser compatibility with the specs.
HTML5 Starter Pack gives you a starting point for HTML5 projects, with a basic framework page, starter CSS, and more.
HTML5 Readiness tries to show which features are relatively wide-spread in browsers and ready to use.
Can I Use? is another take on the current state of HTML5 for production, with compatibility tables for features in HTML5, CSS3, SVG and other upcoming web technologies.
HTML5 Validator tells you if your code is proper HTML5.
The CSS Validator goes to CSS3 now.
HTML5 Test tells you how well your browser supports HTML5.
Peter Paul Koch maintains awesome web browser compatibility tables, and recently he has been extending them to cover mobile browsers.
HTML5 changes how you use H1-H6, essentially each sectioning element has it's own stack of heading elements, starting with H1. Together these all form an outline of your document or page. You can see if the outline is working the way you expect with the HTML5 Outliner.
Some innovative uses of the canvas tag.
Paul Roget on mixing the web at the recent (2010) Mozilla Summit.
Video JS Open Source Video Player
Prezi presentations, quite a different take than the standard slideshow
Akhibahara games in HTML5
Browsers for seeing the future
To test out the features we will all have tomorrow, download and test using the beta browsers from each vendor, a list which currently includes Firefox4 beta, Webkit nightlies, and IE 9 beta. This will let you see how support is coming along for cutting edge features such as WebGL, as well as what currently unsupported bits of HTML5 technologies are likely to be in the next general releases.
All the rest