A few that come to mind for me are
- ASCII Sector - space roguelike that was released in the last 2000s
- Sonic Robo Blast 2 - free 3d sonic fangame, built using a Doom engine of all things…
- ClassiCube - free sandbox block-placing game that is similar to 2009/2010 Minecraft Classic
- Chronicles of Denzar - free browser-based RPG similar to dragon quest (previously PhantasyRPG.com) running since the early 2000’s
https://store.steampowered.com/app/1318690/shapez/
Shapez hopefully counts as “not popular”. It’s a completely chill factory builder (think Factorio with less complexity and zero pressure/resource constraints) whose difficulty continues to increase. Pretty fun.
https://store.steampowered.com/app/960690/One_Step_From_Eden/
One Step From Eden is a MegaMan Battle Network style rougelike. Fun to make builds, but then you get to start again shortly after(cuz it’s hard)
https://store.steampowered.com/app/1062090/Timberborn/
Timerborn is in Beta and is constantly getting more content and QOL updates. It’s a beaver themed city builder to survive droughts.