Noctis

Noctis (currently in release IV) is an open-source space exploration game released in 2000. You play as a Felysian, a cat like sentient species in a Stardrifter, a speedy space ship. The gameplay is mainly exploring the reaches of the universe.

You can find it at anynowhere.com; upon reaching the site, click the hard-to-see "Noctis IV" link along the top edge of your window.

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 * Beautiful Void: Contains 70 billion stars, all procedurally generated. Precious few have life, and your character is the only sentient being. (The website says that there are other exploration ships around, but it's "fantastically unlikely" that you'll ever meet one. For game purposes, you're the only person in the galaxy.) There's no win condition, and no way of losing except against existentialist angst—just exploration, and wonder at the sheer emptiness of it all.
 * Procedural Generation: You explore a galaxy approximately 90 thousand light-years in radius (about double the radius of the Milky Way Galaxy) made of over 78 billion stars, many with planets and moons. If you pick any point in the starfield, you can go there, then land on any rocky planet you find and roam around. All created by a program of under a megabyte.
 * Wide Open Sandbox: In fact, there is nothing to do but explore.
 * Vaporware: Noctis V has been announced, but there has been no sign of development.