Accessible Audio Player uses the HTML5 <audio> element for browsers that support it, and the Yahoo! Media Player for those that don’t. Browsers that support neither degrade to a list of links to MP3 files.
HTML5 Video Player is cross-browser, supports subtitles, is skinnable with CSS and has a Flash fallback for incompatible browsers.
HTML5 Uploader lets you drag-and-drop multiple files for uploading. Supports FF, WebKit and Chrome.
Signature Pad transforms an HTML form and lets users draw their signature on a canvas. Seems like a great way to add some legitimacy to contracts/agreements performed online. Not much different to the electronic signature pads couriers use.
Baker eBook Framework lets you publish books for the iPad that include HTML5 video, audio, animations or more.
Modernizr helps you detect features so you can make cooler web apps.