Unity

Unity is a 3D and 2D game engine allowing good looking games to be made with only one or a few people, Some big companys also use this engine for their games.

3D
Originaly Unity (Then called Unity3D) was only a 3D game engine. The 3D allows for complex shaders, lighting and animation.

2D
Unity Later added a 2D function for people to create 2D games.