Stuff I like: "Physics simulators"

In my case, this is just a fancy name for "powder games".

Check out Powder Game. Simple design, simple simulation, easy for first-time users.
Then take a gander at The Powder Toy. The newer versions are much more complex and offer many options not seen elsewhere (in such a package, anyway). Inside are the normal powder staples- explosives, liquids, plants, virii, and the like- but it also introduces special sorts of walls and electricity. It is, in fact, capable of simulating a nuclear reactor, should you have the ingenuity to build one safely. Bonus points if you don't use "Wall".

I'd recommend the Powder Game for those not interested in installing anything (or navigating a more complex interface), and The Powder Toy for anyone who likes to tinker with physics sims.