Was surprised to see that no one has mentioned the recently open sourced Godot engine, so figured I would add a short post on it. This won't appeal the average C2 user as you need to be able to script. If however, you know your way around Python or can can code in general, you will probably find it very interesting
It has separate 2d and 3d engines, and unlike the recent Unity foray into 2d, it is a true pixel based 2d engine. It is all based upon a hierarchical/node based object system that is driven by scripts created in GDScript(based on Python). It has a built in animation timeline, so you can create cutout animations (like a basic Spriter) or animate any object property on the timeline to be triggered with a script. Also has a 2d and 3d physics system that was created by the devs and is not box2d or Bullet based. The particle system can be previewed realtime in the layout, which is very cool. Unlike Unity, scripting is done in the IDE/GUI itself which I find appealing, but others may not.
As far as I know , there are no library dependencies in the program, which means it is very light . The GDScript is interpreted on the fly, so there is no compiling, yet it is still very fast as it is written in c++. It is very nice to hit play and instantly play the game you are working on. It can export natively to Win, Mac, Linux, Android, IOS, BB, HTML and others (the devs used it for PS3, Wii, Vita, 3DS, but these are not avaialable publicly for obvious reasons).
There are a few rough spots, but since it is FOSS, it will grow in leaps and bounds. It has been developed for 5/6 years and used on commercial games as an inhouse tool up until now. So anyway, if you are looking for a hybrid 2d/3d engine that may eventually give Unity a run for its money, check it out. Oh yeah, it is available on Win/Mac/Linux