On October 21, 2003, a photograph of the nocturnal sky was taken, where almost 2 years later, in January 2005, evidence was raised that there might be a 10th planet at the borders of our solar system: Eris, located in the Kuiper Belt and named after the Greek godess of discord.
On this day in 1994, the last parts of the comet Shoemaker-Levy 9 collided with the largest planet within our solar system, Jupiter. This was the first time, that an extraterrestrial collision of two objects could be directly observed.
Strictly spoken it's a piece of software, simulating the Solar System's bodies in 3D on your Windows or Linux PC (will work in most *NIX's as well). In difference to quite a few other programs it does so in realtime. Meaning you can view all the planets, moons and spaceships move along their paths, trace them, follow them, orbit them and even control them (time and spaceship contol). And you won't have to fight your way through hordes of green, slimey and one-eyed aliens for that ;-)