Adobe has contributed source code from the ActionScript Virtual Machine* to a new open source project, called Tamarin, to be hosted by the Mozilla Foundation. The goal is to build a high-performance open source implementation of the ECMAScript 4th edition (ES4) language specification. The project page and source code will be immediately available as part of this announcement.
* No, Adobe is not open sourcing the Flash Player. ;-P We are contributing source code for the ActionScript Virtual Machine, the scripting language engine component within Flash Player. ActionScript 3.0 runs on the Tamarin virtual machine in Flash Player 9.