Not going to mention classic games this post (Wouldn't do them justice to quickly summarize them), and rather mention two newer games:
Outerra Anteworld: More a tech demo at it's current stage than a game, Anteworld shows a different approach to planetary generation. Instead of procedurally generating a planet it uses map data like osm etc to create a life-sized earth (or, if you're interested in tolkien, there's also a middle earth mod). There's currently not a lot you can do, basically all you can do is walk, drive and fly around an endless grass field filled to the brim with 2d sprite trees. However, as someone who immensely enjoyed Dear Esther, I like to just walk up a mountain while listening to the ambient sounds (there's always a wolf howling every so n so minutes, which I think is nice). bascially virtual hiking.
Subsurfacegames' Survive: Yes, it is yet another zombie survival game. What I find that makes this game mention-worthy (apart from it being developed by a single person who wrote his own engine), is that this game simulates tectonic plates,, how they move etc, to create the continents that the player can play on. Sure, it doesn't matter that much in a game where the main gameplay is to sneak from one house to another house to raid them, but it's interesting to say the least.