Adobe's official IDE for the Flex SDK and AIR SDK. Based on Eclipse.
Prior to Flex SDK 4.0 this IDE was known as Flex Builder.
Flash Builder is designed for developing Adobe Flex applications, Actionscript projects and Mobile Projects. The Flex framework integrates traditional Actionscript 3 code with a tag based language MXML. MXML allows developer to use pre-packaged components that are part of the Flex framework.
This IDE is based on top of the Eclipse Platform (eclipse). It can be downloaded as part of an Adobe Creative Cloud subscription or can be purchased separately.