Game dev environment

Ive been using the blender game engine forever and love it, but I have a few questions:

1)under the world>physics menu there is an option for the physics engine. Used to have ‘none’ and ‘sumo’, and now with 2.40 it has ‘bullet’. I was wondering, can you install other physics engines into blender?

2)Are there any add-ons/ plugins for Blender that aid with game dev? (I suppose some really useful python scripts could fall into this category)

3)is there any other game development software like blender? Just like Blender in that it combines 3d modelling with animation, physics, and programming it all into a game?

Anything like these would be awesome. So do they exist? thanks