Reality based simulations seem to be the in thing right now. Last month we mentioned the X-Plane Space Combat game and the Orbiter simulation. Now TheRedDuke has brought to our attention Celestia. Celestia is a free, open source universe simulation which enables you to explore the solar system and 499 other stars in 3D, in real time. There is also the Motherlode site which offers a whole load of additions to expand on the program's features. It's pretty fun travelling around things like Hubble and Huygens too. You can download Celestia in Windows, Mac OS X and Linux flavors here.