http://pushbuttonengine.com/PushButton Engine is pretty much the only one I know of. I think the basic idea of it is to provide a "framework" that allows you to write "components" or basically wrappers of engines like Box2D or FLINT to work seamlessly together in the engine.
How well that works I can't really say, but it should have you covered for 2D game needs.
It IS free (and I think partially Open Source) but there will be some components later on (like networking) you'll have to buy for money.
The engine IS pretty new. It just finished its closed Beta and was released onto the public just this month.
I personally found it hard to get it to work with FlashDevelop/Flex in the Closed Beta (in fact, I didn't manage) but they have a tutorial for that up now, which I haven't tried out yet (it's under Documentation).