The first ever computer visual that really captured my attention was the old Windows screensaver where the logo bounces around the screen. Naturally, I wanted to recreate that, and, naturally, I wasn't happy with the initial result of a single circle just moving about.
So what did I do? What every sane person would do: I added more stuff and made some freakin' black holes that you can put wherever you want.
Click to create a black hole
(click on the gear in the top right)
Pull, Push and Twist are the three black hole modes
Reset re-runs the program
Freeze freezes the scene
Infinity causes black holes to stay forever; they never shrink or die