Dive into HTML5 is a website by Mark Pilgrim that “seeks to elaborate on a hand-picked Selection of features from the HTML5 specification and other fine Standards”.
The site is based on a book that has been published by O’Reilly: “HTML5: Up & Running“. It contains many useful tutorials, and an excellent introduction section on HTML5.
UPDATE, 11/10/2011
Apparently, last week Mark Pilgrim, or someone on his behalf, took down his websites from the web. There are a few alternative ways to still access Dive into HTML5, in places such as:
http://diveintohtml5.info/
http://mislav.uniqpath.com/diveintohtml5/
http://mislav.uniqpath.com/2011/10/dive-into-html5/
https://github.com/diveintomark/diveintohtml5
And a bit more on the story, with more links:
http://meyerweb.com/eric/thoughts/2011/10/04/searching-for-mark-pilgrim/