I'm sorry, I couldn't resist that title. Phun is a "2D physics sandbox" where your students can have fun while learning.
The creators of Phun describe the program like this:
Great for kids
Phun is a fantastic toy for children, where they can learn and
appreciate physics, science and simulations in an open ended gameplay
with rich creative and artistic freedom, including colorful freehand
Tina, a FreelyEducate.com reader, sent in the link to Phun. She said she appreciates why the creators are keeping Phun free: "We want to keep Phun freely available for non-commercial use, since
we think and hope that Phun can make a difference in promoting science
and art to the masses, in particular children and young people, through
a constructionists learning paradigm."