Escape Velocity: Nova - One of the all time classics of open world games being a space trucker that gets wrapped up in galactic events. Sort of unknown because it came out on Mac at first, then was ported much later, then the studio behind it vanished like a soap bubble. The fan community maintains their own set of download links

Star Control II / The Ur-Quan Masters - Open world space RPG that's not quite like anything else. Kind of unfair and old-school (take notes, save often, and rotate those save files), but really a wild story of interlocking open pieces, and fantastic character writing. Not quite abandonware, but the devs open sourced it (The Ur-Quan Masters is the open source name). If you're on Linux, it's probably available in your distro's software center.

Cave Story - Excellent indie story-platformer from just before people started being able to make money off those things. Was originally free, then was given an "upgrade" (made worse) by a company that royally screwed over the original dev. The fan community maintains their own set of download links including a modern engine rewrite that works on modern systems. (If you want to actually pay for this, don't, and instead buy Kero Blaster)