Check it out: OpenPyro: OpenPyro is a pure AS3 framework for creating RIA’s. Open Pyro draws a lot of inspiration from Flex but aims to be more expressive as well as have a smaller filesize and memory footprint.
Arpit Mathur, one of the most brilliant developers I know and a straight up Flash guru is leading the Open Pyro project. He recently posted about OpenPyro on his personal blog and includes a screencast of him using the framework to develop an app.
Kevin Fitzpatrick another CIM Flash master, and lead developer of another open source project at CIM, LogBook, comments about OpenPyro.