Dwarf Fortress
Dorf Fortress (AKA Dwarf Ortress, Dorf Ortress, Dor Fortress, Dwar Fortress, Door Fortress, Dwar Ortress, Dor Ortress, Dorf Fort, Dwarf Fort, Dorf Ort, Dwarf Ort, Dor Fort, Dor Ort or Dwarf Fortress) is the best game in the world. It was created in the dawn of time by Toady One. The ostensible objective of the game is to manage a dwarven fortress, but the usual result of playing the game is hilarious failure.
Unlike most sane games, Dwarf Fortress does not actually have a winning condition. Every fortress, no matter how successful, is doomed to a hideous death at some point - in fact, in older versions of the game, the simple act of mining a certain extremely deep and rare ore would start a hidden timer condemning your fortress to certain destruction at the hands of a balrog standin, with the game sadly informing you that your dwarves dug too deep, but keeping your fort going long enough to strike that ore was an achievement in and of itself. This inevitability has lead to the fan base's rallying cry: "Losing is Fun!" In fact, in discussions on the topic, the word 'Fun' (especially with capital 'F') is entirely synonymous with 'Hideous Demise' and the things that are likely to cause it, with 'Hidden Fun Stuff' used to refer to the demonic late-game enemies and Hell itself.
The gameplay has an exceptional and frankly obsessive depth of detail. Despite being (by default) ASCII-based and extremely obtuse, like the old roguelikes from which it draws inspiration, huge amounts of information are tracked and considered for just about every aspect of the game - down to minute details such as the exact location and severity of injuries (first joint on left little finger slightly bruised, for example). Combat is complex and messy - a typical dwarven battlefield will be full of bloody stains, severed limbs, discarded weapons and crossbow bolts, and the vomit of the unforunate recipients of abdominal injuries. After-action combat reports give detailed and often hilarious or epic blow-by-blow accounts of the fights that take place, and the player even has the option of entering adventurer mode to explore their world and get in fights themselves, presuming they enjoy being shot by archers off the edge of the visible area.
Dwarf Fortress is still in alpha and under development, kept going solely by donations from fans. The official game's ASCII-based display of inscrutable letters and symbols confuses the shit out of fucking casuals, but fortunately an unofficial tile graphics version is available here. However, it does have a few minor quirks since the actual game does not yet fully support tile graphics.
Posting a Dwarf Fortress thread on /tg/ is a great way to effortlessly troll a few people, confuse others, and cause multiple, simultaneous and devastating orgasms in neckbeards.