Universe Sandbox 2.
Universe Sandbox 2 is a physics-based space simulator that merges gravity, climate, collision, and material interactions to reveal the beauty of the universe and the fragility of the objects that inhabit it. You can create, destroy, and interact on a scale you've never before imagined. You can see the effects of gravity on objects in the universe and run scale simulations of the Solar System, various galaxies or other simulations, while at the same time interacting and maintaining control over gravity, time, and other objects in the universe (moons, planets, asteroids, comets, black holes, etc.).
After watching the interplay of our own solar system for a while, and not wanting to wantonly destroy our own (simulated) system, I created a solar system from scratch and then threw a wrench in the works by inserting a small black hole in the neighbourhood. Then I sat back and watched things fall apart. The result wasn't what I expected, as gravity wells don't quite behave the way you'd think they would. Thus is the life of an amateur deity.
What's nice about these simulations is that you can save them to load up again later. It's like putting the universe on pause (also doable) and getting back to the excitement only when you're in the mood.
After watching the not quite so destructive interplay of sun, planets and black hole, I decided to load up a preset planetary collision. BOOM! You not only get to see the initial collision, but also the secondary (and more) collisions of matter that is blasted off the planet's surface, eventually gets captured by gravity and reintroduced to the planet's mass in the most spectacular way. In case you wondered how the dinosaurs went extinct, you could simulate that in this program. This simulator could keep me entertained for months.