Flex
Adobe Open Source
Adobe has one of the strongest open source libraries of any of the top publicly traded software companies. You can get Flex, Blaze Data Services, the Tamarin Javascript compiler, the Cairngorm Flex framework and more.
Help Open Source Flex SDK win the SourceForge Community Choice aware
Why not help the free Flex SDK win the 2008 SourceForge.net Community Choice Award for Best Tool or Utility for Developers? You need to setup a SourceForge account but you can easily do so using OpenID.