I like simutrans a lot (maybe the most elaborated TTD-like transport game), and I am trying to learn freeciv, but it is a bit "technical". I also like minetest, I have constructed a lot of big houses, the only FOSS game where you can cook a lot of recipes.
My personal favourite is Dungeon Crawl Stone Soup, although it's quite tricky to get into and still brutally difficult, even if you are in it.
It's similar in concept to Shattered Pixel Dungeon, which others have mentioned here. Maybe try that one first. ๐
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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