I especially like the orbit simulator because I once had to program one in university. During my third semester (I was still aiming for a master in physics back then) I took a course on theoretical mechanics, and we had to write a program that simulated the paths of the moon relative to the earth and the sun. The masses and velocities of the objects were adjustable.
It took me weeks and weeks to finish because I had never programmed anything serious before, but it still didn’t calculate correct paths the day before the deadline, so I had to ask for some more time. Two frustrating afternoons later, I finally found the sign error in one of the formulas… Sigh.
I just realized that this must have been in February of 1996, so more than a decade has passed since then. I guess they do different programming tasks in theoretical mechanics now that you can just download applications like this from the web. Which is a good thing for me because now I can use these spiffy simulations in my lessons instead of my amateurish ones. I’m sure the students are going to appreciate it.