Flex 3 is the Adobe solution towards RIA on a cross plataform level.
But is this Flex thing Open Source?
There are 2 licenses to consider for the various packages of the Flex SDK.
|Mozilla Public License, version 1.1 (MPL)||Primarily covers the source code of the Open Source Flex SDK including the Flex framework, the compilers, and the debugger.|
|Adobe Flex SDK license||Covers elements that Adobe was not able to release under an open source license including the Adobe® Flash® Player, Adobe® AIR™, and other third-party code used by the Flex SDK for specific functions, such as font encoding.|
So if you are using the Open Source Flex SDK, you are entitled to the terms of the Mozilla MPL licence (seen above for details). We’re just concerned about using the Open Source Flex SDK rather than the official products of Adobe, which are bundled to their licence because they contain non-open code (AIR and the Flash Player mainly…).
Downloading the Open Source Flex SDK
Current version (stable) is Flex 3, with the newborn (Gumbo) on its way soon. The download page is this.