I tend to focus a bit on the 3D side of things, but the recently open-sourced Torque2D (note the "2") engine is pretty cool too:
And in fact it got a whole lot better in the last couple of weeks with it being ported to Linux, Android and your browser (through Mozilla's emscripten).
So if you are thinking about developing an open-source 2D game targeting multiple platforms, Torque2D has just became a serious contender.
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