Finally actually tried Celestia. Frickin wow.

Favorite thing to do so far... Turn on Galaxies, center on the day side of Earth, and start zooming out (both mouse buttons, drag back). Keep on zooming out until you're looking at the whole Milky Way.

Also fun... Turn on star labels and go star hopping. Select a star and use G to go to it. Do this for a while until you're lost in the galaxy, and then try to find your way home. Hint: Sirius and Vega are both pretty near Sol...