Right, I'm busy today, so just a quick link to be going on with.
Well, I say a quick link, but it looks like you could spend hours at galaxiki.org, playing God with fictional solar systems.
I had a little nose around at the weekend, but I didn't play with any of the planet customising features or anything.
Unfortuantely the rules prohibit the use of 'names, characters or stories from exisiting science fiction stories on Galaxiki solar system pages' otherwise I would have been happily creating the planets from Countless as the Stars. It is quite possible, of course, that that rule applies to naming a planet Vulcan or Viltvodle VI, and that the planets I created myself might be allowable. But I don't suppose they want me imposing my Creed on their nice shiny galaxy.
Anyway, if you've got too much time, and want a new kind of world-building tool, go have a look.