PushButton Engine is an open source, flash game framework from PushButton Labs. We use it in our own products, along with industry leaders like Zynga, Playdom/Disney, RockYou, hive7, and others. In total, PBE has powered hundreds of millions of game experiences.
PBE is designed to make it easy to integrate existing libraries and build gameplay on top of them. Check out the documentation for more details. PBE comes in two flavors: PBE 1.0, the classic battle tested option, and PBE 2.0, the new, lightweight, and refined version.