ConcernedApe & Chucklefish: 1st is creator of Stardew Valley, 2nd is creator of Starbound; two games I enjoy modding.
Super unsure if they're a full blown company or not, but they're the two I used to follow development for frequently.
I also was super into Klei a while back, but sadly don't play Don't Starve Together as often as I used to (old friend group died off).