These days, my son is playing with these (apart from usual game sites):
Alice is a 3D programming environment. He likes it as much as he likes Scratch.
Interactive simulation tool for physical phenomena from University of Colorado.
He kept trying to soft land "on the moon" :-) I guess Chandrayaan I has impressed him a lot!
What is the common between Alice and PhET apart from being great education tools? It is Java! With the advent of JavaFX, we can expect that such fantastic rich GUI applications will be written in JavaFX.