MacromediaFlex (now AdobeFlex) is a product available since 2004 that will allow developers to generate an SWF front-end to their applications, using XML (called MXML) for structure, ActionScript for events and CSS for style. The development SDK is free of charge, but Adobe sells an Eclipse plugin called FlexBuilder? that provides a GUI form builder and other nice stuff. A subset of the free SDK without the Flash Player or AIR is available under an open source license.
How this product will influence the future of web application development is anyone's guess. (Are RichInternetApplications partially the result of the influence of products like this?)
http://www.macromedia.com/software/flex/?promoid=home_prod_flex_111703
You can see beta examples and feature discussion:
Christophe Coenraets - http://www.markme.com/cc/
Ben Forta - http://www.forta.com/blog/
Resources
product overview at http://www.macromedia.com/devnet/flex/articles/flex_overview.html
Building RichInternetApplication ... at http://www.macromedia.com/devnet/flex/articles/flash_perspective.html