Adobe has released Flex 3 and AIR (Adobe Integrated Runtime) to the world. Flex is an open sourced framework for building better RIA's (Rich Internet Applications), and both Flex and AIR have free SDKs.
Flex doesn't need a "Flex player" since it runs in the Flash Player that so many of us have.
There are tons of Flex-based sites and applications running powerful front-ends, here's the Flex showcase: http://flex.org/showcase/
If you're interested in learning Flex then check out http://flex.org and information about the AIR desktop runtime can be found at http://www.adobe.com/products/air/ or downloaded directly at http://get.adobe.com/air/ .
You can make Flex applications with professional IDE's or with a text editor. You can make AIR apps the same way, or in Flash, Flex Builder, Aptana, or even Dreamweaver. You can make AIR apps our of pure JavaScript. AIR does require an AIR installer, but then your apps can run like any native app for Windows, Mac OS X, and Linux desktops.
Drupalcon Boston 2008: I'll be doing a presentation on Flex 3 with Drupal, titled Better Drupal Interaction Design with Flex.
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30onAIR
Videos from various people around the world who are helping answer the questions - Why Flex? Why AIR? http://www.30onair.com
Chris Charlton, Author & Drupal Community Leader, Enterprise Level Consultant
I teach you how to build Drupal Themes http://tinyurl.com/theme-drupal and provide add-on software at http://xtnd.us