While we're at it, just for the absurdity: Google Earth is also open source (albeit without planetary or maps data so it doesn't really fit here): https://github.com/google/earthenterprise

Contemporaneous to Keyhole which became Google Earth, there was an open source project called WorldWind:

https://worldwind.arc.nasa.gov/

It's changed a lot over the years, but I figure it belongs in this thread no matter what form it's presently in.

Wow. I forgot about Keyhole.

A place I worked from 1999 to about 2002 had what they called a "Keyhole machine." I'm not sure if it was supplied by Keyhole, or it was just the company's machine with Keyhole's software on it, but part of my job was to produce three to ten maps each day for use on a broadcast television station, and it was an incredibly cumbersome process to get the maps from the computer onto videotape (Betacam, IIRC).

And there is also Universe Sandbox 1 and 2: http://universesandbox.com/

Universe Sandbox has an awesome UI!

I think it is custom built tho, but I would love to see how they did it

There is a podcast by Zach Barth (of Zachtronics) where Universe Sandbox is discussed at length. If I recall, its an entire episode.

Edit: yes, episode 4. http://www.zachtronics.com/podcast/

Celestia is old now but still brilliant

Yes, it's great. Unfortunately it doesn't have an abstracted-out renderer, there are deprecated immediate mode OpenGL calls all over the place, and moving the rendering code to more modern OpenGL is hard. It's being done, though.