Apache Flex, formerly Adobe Flex, is a software development kit (SDK) for the development and deployment of cross-platform rich Internet applications based on the Adobe Flash platform. Initially developed by Macromedia and then acquired by Adobe Systems, Flex was donated by Adobe to the Apache Software Foundation in 2011 and graduated to become a top-level project in December 2012.
The Flex 3 SDK was released under the open source Mozilla Public License in 2008. Consequently, Flex applications can be developed using standard IDEs, for example Eclipse, as well as the proprietary Adobe Flash Builder. The latest version of the SDK is version 4.9. It is released under version 2 of the Apache License.
Read more about Adobe Flex: Overview, Notable Sites Using Flex, File Formats