If you're considering building full-blow rich application on the web, you need to consider SproutCore.
<p>This is a hands on session. We'll be building a SproutCore application on the stage and deploying it to Google App Engine before the session is over. You will also learn about the new SproutCore 1.0 API and major performance enhancements. We'll also show you a preview of some of the next generation technologies planned for later releases.</p>