Algodoo is a 2D physics simulator which aims to be fun, easy to use and educational.
The program offers simple drawing tools like boxes, circles, gears, brushes, planes, ropes and chains. You can select something and click and drag to add it, just like any other drawing tool.
These aren't just static shapes, though-- they're real objects in Algodoo's 2D world. You can interact with them, from bouncing a circle on the floor, to rebounding one rectangle off another, and linking objects together for more sophisticated interactions.
The more you explore, the more options you'll discover. You're able to add fluids, springs and hinges, play around with optics and lenses, alter gravity, friction, attraction and more.
You don't have to get into this kind of complexity immediately. The program has an "Algobox", an online repository of scenes that other people have created. In a click or two you can be viewing examples like roller coaster simulations, a "Tower of Hanoi" scene, an escalator, models of different objects, games and more. Click anything appealing and it downloads immediately, ready to explore further.