If you have to go with software only for some reason, Squeak should be nice. Also VPython (http://vpython.org/) allows one to animate 3D-scenes easily. PyGame (http://www.pygame.org/news.html) might also be worth a look, however it is slightly more difficult to get started (the level of abstraction seems to be a bit lower, as you handle sprites yourself).
If you have to go with software only for some reason, Squeak should be nice. Also VPython (http://vpython.org/) allows one to animate 3D-scenes easily. PyGame (http://www.pygame.org/news.html) might also be worth a look, however it is slightly more difficult to get started (the level of abstraction seems to be a bit lower, as you handle sprites yourself).