Archipelago has Multiworlds (I'll call them MWs, they're called Rooms) which themselves contain slots (numbering 1 or more) which all correspond to a specific game. Within MWs, all games and their items are placed in a central pool, and then reshuffled back to all those games. Players will play their slot(s) as normal, but when they find items, it will potentially be something for themselves, or (more likely in high slot counts) it will be something for others. OOT players could find Cinders of a Lord for Dark Souls 3 players.
It's a Randomizer but on the scale of multiplayer and with support for multiple games. Currently there are only 65 "supported" games for Archipelago, and there are plenty more in the works, all supported by the community's developers as it's an open-source project. It also allows you to host Multiworlds and generate the randomizer aspect locally.
Some unsupported games people are working on or at least interested in seeing include: Duke Nukem 3D, Ultrakill, Psuedoregalia, Civilization 6, Metroid Prime, Grim Dawn, Kingdom Hearts Dream Drop Distance, Mindustry, Rimworld, Cult of the Lamb, Diablo 1, Final Fantasy 12, Chrono Trigger, Monster Sanctuary, Ori and the Blind Forest, and there's even Archipela-Go!, a game being made specifically for Archipelago, among which are much smaller Sudoku, Clique, CheckFinder, and Bumper Stickers which are varying sizes of minigame.
There's also an After Dark community server for supporting and playing +18 games (according to Pegi ratings which ofcourse vary wildly in quality) with Archipelago, though none currently exist that I would explicitly call +18-material.
