I teach astronomy, and a student just told me about Earth Editor. It's from
Dan-Ball, a free game site.
dan-ball.jp/en/
If you click on Earth Editor, you can simulate building the Earth.
(Interesting to me, since I just taught about the Earth.). For example,
click on SAND, move cursor into the black field, then click the cursor and
move it around. The central white dot gravitationally "attracts" the
particle (sort of like how protoplanets form). Not sure how accurate the
physics/forces are, but I thought it was a cool simulation. Wanted to share
it with the group.