HTML 5

What's new in HTML 5

The new features:

<video> and <audio> tags for media playback (say goodbye to Flash players);
structural tags <article>, <section>, <nav>, <aside>, and <footer> (used instead of <div class="section"> and <div id="footer">; the benefits are improved recognition of the page elements by search engines and screen readers);

new control types for web forms (email and URL fields, color and date picker, spinbox, slider, progress bar, meter, and key pair generator);
client-side form validation: required fields, regular expression constraints, minimal and maximum values (previously was done with JavaScript or AJAX);
placeholder text and setting focus (JavaScript was used for this purpose, and it often was buggy);

<menu> tag for creating hierarchical menus, toolbars, and context menus;
client-side storage accessible from JavaScript;
Geolocation API for tracking user's location from GPS or Wi-Fi data (test in your browser; only Firefox works now);
standardized attributes for drag-and-drop, WYSIWYG editing, and spell checking;
simplified <!doctype html> and <meta charset="..."> tags.

SketchPad. A drawing web app using HTML 5 canvas (no Flash required).
Vaullt. HTML 5 <video> showcase. See also page source (<nav>, <aside>, and <footer> are used for structural markup).

Web Design Checklist. HTML5 and CSS3 support in different browsers. Also in your current browser (scroll down one screen on their site).

Can I use: compatibility tables for HTML5/CSS3 support in desktop and mobile browsers. They show where can you use a particular HTML5 feature.
The HTML5 test. Check how well does your browser support HTML5.
The official standard. See also a shorter version for web authors, another shorter version, and a non-normative document on markup.
New tags, attributes, and events.
Wikipedia article on HTML 5.
WTF is HTML5 (infographic).
Dive into HTML5 by Mark Pilgrim (e-book).
HTML5 Doctor, helping you implement HTML5 today (blog).
HTML5 presentation: new JavaScript APIs, HTML5 tags, CSS5 selectors and attributes. The site works only in Google Chrome :(