WYSIHTML5 Is a lightweight rich text editor that can generate valid and semantic HTML5 markup.
Fixie is a great way to add in the Latin filler text based on the interpreted HTML5 tag used.
Sisyphus uses local storage to save your existing form input in the event of a browser crash.
Kinetic JS aims to make content creation and manipulation in the canvas a much easier and faster experience.
Turn.js is a cool library that can create the page turning effect on your divs.
Resumable provides a great way to upload files and maintain stability even with lost connections.
Pxloader can provide you a way to preload HTML5 elements and assets before you need to take a specific action.
CanvasLoader It is a free online tool to help you generate scripted preloaders.
HTML5 Video Voting is a neat library that you can implement to allow voting on specific parts of a video.
Buzz gives you more control over the HTML5 audio element.
Blake is the lead developer at YouDesignIt, a t-shirt printing company. He has recently been tasked with making the conversion from Flash to HTML5 and has found the project quite challenging. You can follow him on Twitter at @youdesignit.