it's called crayon physics deluxe. it's fucking incredible. there's shitloads of games that copied it on the internet now, but this is where it's at, the original.

http://uk.youtube.com/watch?v=1JuG7wUByq4&e
(might want to turn the music off it's terrible)

now, is that amazing or what? you can draw whatever you like and it'll come to life. just like that.