>Race into Space
Race to the moon between the USSR and US. A management sim that is adapted from a board game, and with an interface that was considered shitty in the 90's, it's still pretty good. Free.
raceintospace.org/
>Buzz Aldrin's Space Program Manager
Race into Space's younger, slightly worse cousin. Shinier and slightly better interface, though, which might be a selling point for some. It's a Slitherine game - not much else needs to be said.
store.steampowered.com/app/308270/
>Orbiter
The big boy spaceflight game. Simulates lots of real and fictional spacecraft. Absurd numbers of mods to add just about anything you can imagine.
orbit.medphys.ucl.ac.uk/
orbithangar.com/
>Space Engine
Space screenshot maker with an entire procedural universe to fly around. Is supposed to someday get actual gameplay. Free.
en.spaceengine.org/
>Rogue System
DCS in space. In early access, so approach with caution, but seems to be shaping up pretty well so far.
imagespaceinc.com/rogsys/
>Children of a Dead Earth
Claims to be a "Realistic" space combat simulator. Not really true, and some consider the lead dev to be kind of an idiot, but still an interesting project nonetheless.
childrenofadeadearth.wordpress.com/
store.steampowered.com/app/476530
>House of the Dying Sun
Space anime combat villain sim.
store.steampowered.com/app/283160/
>Rodina
Like an itty-bitty No Man's Sky. Lots of exploration in a single solar system. Early Access, so take caution.
store.steampowered.com/app/314230/
>OpenTTD
A transport and logistics systems simulation game. Free Transport Tycoon remake with tons of addons to keep you busy for the rest of forever.
openttd.org/en/
>Project Highrise
Think Simtower with more stripped down gameplay elements. More casual for sure, but still plenty of fun.
store.steampowered.com/app/423580/
Off-topic Discord:
discord.gg/nUy5x67