If you like Crayon Physics, you'll probably also like Phun.

Youtube video:
http://www.youtube.com/watch?v=0H5g9VS0ENM

You can download Phun for free at
http://phun.cs.umu.se/wiki

Basically the same concept. Didn't have a closer look at this Crayon Physics stuff, but I think Phun is even more powerful as it also supports particle/water simulations.

Though I wouldn't hold my breath to see Phun on the NITs soon, as (at least with extensive particle simulation) it can even bring my 3 GHz P4 down to it's knees.

Edit: just saw, that Phun is already linked in the first post of this thread.