Algodoo

you can play with simple sandboxes and create your works from monotone parts set.
example sandboxes are:
- gears and links ... move gray boxes, you'll find links draw various figures [with move tool]
- crawlers ... bump crawlers [with left or right key]
- slingshot ... shoot balls at the blocks [with hand tool]
- grappler ... catch balls moving in the room [with hand tool]
- marble run ... drop a ball with movable bars [with hand tool]
- worms ... watch crawling worms [with up or down key]
- water cycle ... watch water circulation. (that's not perpetual machine because use 'heater' which is the sun in nature. if you lift the right side bar up, balls (= water) aren't warmed up never and stop the cycle forever.)