This list will keep expanding over time!
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Dwarf Fortress Free
Simulation game the complexity of which is astounding. Watch this video if you're curious about it, and if you want to play, I strongly recommend using a Lazy Newb Pack and looking up a tutorial.
"Unusual mix of an adventure and an idle-game. The player controls a Shade, who is told to keep watch of a sleeping King for 400 days until he awakens. The catch: These 400 days start to count down in real-time. The player will be completely on his own, in a giant underground cavern. Do you simply want to wait? Do you want to build a home for yourself? Do you want to find a way out of the cave? Over time, parts of the cave change– moss and mushrooms grow, spiders craft their webs, water is dropping from the ceiling, stalactites break and fall. It's a game about loneliness, escapism and the eternal search for purpose".
One of my favorite games ever, never seen anything similar to it. I recommend going into it blind, without looking up guides. But if you must do the latter, here's a hint guide that won't spoil the game.
Ib [English translation] Free
Ib [Original japanese version] Free
"A young girl named Ib visits an art gallery with her parents. While observing the many exhibits, she suddenly realizes she is alone. And in her search for others, she finds things awry in the gallery...".
Despite its graphics, which are not my favorite, it remains one of my treasured games for how it does atmospheric, creeping horror. I also enjoy its music and art gallery theming.
UnReal World Free
The first game of the survival genre that has also been in active development since 1992, continuing to share updates as of 2021 (three decades!). It's a 2D game that's heavy on text and commands (you can control it entirely through the keyboard), and it offers an incredibly realistic experience compared to other games of its genre, plus a very educational in-game encyclopedia. You can craft tools, traps and buildings; use a plethora of skills such as cookery, agriculture or even weatherlore; interact with NPCs and their culture or be a hermit; and you need to keep an eye on many things such as hunger, thirst, the load you carry, temperature, wounds, exhaustion... The version just before the latest one gets released for free.
"Explore procedural galaxies, land on unknown planets, read the thoughts of every living thing, rummage through people's pockets, and more!". Text-based, online.
Yume Nikki Free
The entirety of Yume Nikki takes place inside the protagonist's dreams, full of surreal creatures and vast locations. There is no combat or quests besides collecting different "effects", which the player can use to advance to new areas. It has inspired multiple fan games based on the same premise.
It is a favorite game of mine for achieving a desire for exploration that I've seen in few other ones— probably thanks to its weirdness and its bare-bones walking simulator mechanics. It can be eerie, scary or sad, but also calming.
Following the link, under "Download options" at the right, there's a ZIP for the English translation as well as the original Japanese version.
"Digital publication focused on speculative biology, art, and science fiction". Biannual.
All Tomorrows: A Billion Year Chronicle of the Myriad Species and Mixed Fortunes of Man, by C. M. Kosemen (2006). Free
Speculative biology (a genre of science-fiction) story. From its Wikipedia article: "It explores a hypothetical future path of human evolution set from the near future to a billion years from the present. Several future human species evolve through natural means and through genetic engineering, conducted by both humans themselves and by a mysterious and superior alien species called the Qu". There's also a great abridged narration by Alt Shift X.
About research and unanswered questions in biology. "Scientists talking to scientists, but accessible to anyone".
Welcome to Nightvale
Inmersive fiction podcast in which you listen to the community radio of a desert town where weird, strange events are a part of everyday life. The radio host's narrates the weirdest news, advertisements and announcements from the fictional town with his very relaxing voice, as if they were mundane, which makes for a very interesting mix of deadpan humor, surreal horror and drama. It also does well in terms of representation of oppressed groups.