Swedish graduate student Emil Ernerfeldt created the program Phun, a 2D physics playground, and has made it free to download for non-commercial use.
He demonstrates it in a zenful YouTube video, where he creates devices like cars and piston engines in seconds using simple shapes.
Download app here.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects