Dive Into HTML5.
Dive Into HTML5 probably offers the best balance of depth and accessibility to the would-be HTML5 developer that you can find online for free. Mark Pilgrim has been writing about HTML and Web development for several years. The "paid version" of this book is HTML5: Up and Running from O'Reilly.
HTML5 Doctor has been publishing tutorials for the past couple years. The site is well organized and includes a section on getting started, and several others that focus on specific features.
SitePoint's HTML5 and CSS3 Tutorials
You can download a free PDF sample of HTML5 and CSS3 for the Real World by Estelle Weyl, Louis Lazaris and Alexis Goldstein from SitePoint, but you'll have to swap your e-mail address for it. Much of the same free material can also be found in the site's HTML5 and CSS3 sections. But unlike HTML5 Doctor, you'll have to dig around a bit.
One particularly helpful tutorial is this one on HTML5 and CSS3 in Internet Explorer 9.
W3C HTML5 Reference
If you want to know everything that's in new in HTML5, you can check out the W3C's HTML5 reference site, an extensive source of information on all the HTTML5 elements.
You can also check out the current draft of the spec.
HTML5 Cheat Sheet
The HTML5 Cheat Sheet is a free PDF features a quick reference guide to HTML5