Adobe Flex is one of the pioneers and leaders in the Rich Internet Applications (RIA) space, and with the advent of Adobe AIR, it has proved itself as a viable option for desktop and mobile applications as well.
Here's how Adobe describes Flex on http://www.adobe.com/products/flex/:
"Build engaging, cross-platform rich Internet applications
Flex is a highly productive, free, open source framework for building expressive web applications that deploy consistently on all major browsers, desktops, and operating systems by leveraging the Adobe® Flash® Player and Adobe AIR® runtimes."
Flex is a rich framework that lets you build rich applications for deployment on the Internet as well as desktops and devices. Flex ...