How about using Citus Engine? It's free and easy to use. It's ActionScript 3.0 and can be exported on various devices through AIR technology. Also it takes advantage of Stage3D for high performance 2D graphics (Starling API) or 3D graphics (Away3D API). It has many tutorials and free examples and also has some built in platformer functionality to help you get started quickly.
I really like Citus Engine , thanks for share !