Just Unity. All the rendering part is handled inside Unity itself, using OpenGL (not quite sure about that one !) without you seeing it. If you can use OpenGL functions in scripts when need, it's quite rare. But you don't "juste need Unity", unless you want a game made of boxes, spheres and capsule :p
So it's at least Unity + 3D modeler + 2D software.
answered Jan 28 '12 at 12:20 AM