Our Worlds

Celestia

Celestia... Space simulation that lets you explore our universe in three dimensions.
Celestia runs on Windows, Linux, and Mac OS X.
https://celestiaproject.space/
Downloads for each OS:
https://celestiaproject.space/download.html
Addons:
http://www.celestiamotherlode.net/
A (not so) Brief Introduction to Celestia Addons:
https://www.classe.cornell.edu/~seb/celestia/addon-intro.html