Here's my three. Two aren't exactly unknow, but are far from mainstream, the other may be a bit more obscure.
First up is Kerbal Space Program.
I think it's still in Alpha, but it is steadily approaching Beta, with constant solid progress. It's being developed by a Mexican graphic design company. The lead developer went to his boss one day, asked if he could make a game, and after presenting a his idea and a business plan, he got the go ahead. The team keeps expanding and it's now an awesome space sim.
https://kerbalspaceprogram.com/Second is the most obscure of the three, Blockade Runner.
Kinda like Minecraft in space, I haven't played it for about a year, but I've been a backer for maybe 18 months. It appears to still be under decent development.
http://blockaderunnergame.com/And for all the masochists out there, I give you Dwarf Fortress:
http://www.bay12games.com/dwarves/Ok, so it looks batshit boring. It's not. It does, however, have a really steep learning curve. Seriously, if you're going to give Dwarf Fortrss a try, get Dwarf Therapist and follow a tutorial. You WILL NOT enjoy it at all if you just try to wing it. Follow pretty much any tutorial, then once you have a bit of a clue what's going on, use the wiki to find out specifics.
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