A free passion project from a team of game devs has released on Steam after a few years in Early Access, and it's entirely free. Planet S is a game where you're dropped, with your single colony ship, into an uninhabited solar system either alone or alongside others with their own ships. From there, it's your job to try and expand to get the resources you need to survive and thrive.
Starting either by yourself or with other players, you then go about grabbing a starting planet with the necessary resources to expand. From there you'll eventually hit a resource not on your starting world, requiring you to expand to one of the other worlds in the system.
It's not the fanciest game, to be clear. It's a pretty simple RTS without a ton of depth in combat, but it does use a kind of Settlers or Anno style resource rate system to have you supply your civilization—which is pretty neat once combined with the mechanics of inter-civilization trade, expansion, and combat.
Probably the coolest part is that you'll have to continually support new resources to upgrade your citizens to a higher tier. Higher tier population unlocks new, more advanced buildings and produces more research, used to upgrade your existing infrastructure to be more efficient and to unlock yet more new facilities.
So get out there, grab a few buddies, and go to town on trying to conquer a solar system. The high range of customizable parameters in map generation will let you tweak it to be brutally rough, with lots of limitations, or a resource-rich setting where you'll be throwing battleships at each other in no time.
You can find Planet S on Steam for free.