Adobe's Overview of Flex free felx3 SDK

"Flex is a free, open source framework for building highly interactive, expressive web applications that deploy consistently on all major browsers, desktops and operating systems. It provides a modern, standards-based language and programming model that supports common design patterns. MXML, a declarative XML-based language, is used to describe UI layout and behaviors, and ActionScript 3, a powerful OO programming language is used to create client logic. Flex also includes a rich component library with over 100 proven, extensible UI components for creating RIAs, as well as an interactive Flex application debugger.

Rich Internet applications created with Flex can run in the browser using the ubiquitous Adobe Flash® Player software or on the desktop on Adobe AIR™. This enables Flex applications to run consistently across all major browsers and across operating systems on the desktop. And using Adobe AIR, the cross-operating system runtime, Flex applications can now access local data and system resources on the desktop"

FREE ADOBE FLEX 3 SDK
You can get the complete Flex framework separately from Flex Builder 3 by downloading the free Adobe Flex 3 SDK. Developers can create complete RIAs using only the free Adobe Flex 3 SDK. The Flex SDK is also available as open source in the Open Source Flex SDK project.