Space Engine is a free (but not open source) program that allows you to fly through vast reaches of the universe. Along the way, you'll see some pretty amazing vistas and probably want to take screenshots of them. It incorporates a good amount of real-world data about the solar system, exoplanets and the universe in general with procedural generation of everything we don't know. [more inside]
3D Starmaps by Winchell Chung. (I knew him for his game illustrations before I ever knew about his starmaps.) The site contains lots of information about how to make 2D/3D starmaps from standard star tables, a nice selection of pre-existing maps and one of the best listings of 3D starmap software around.