Videogame wikis (general)
Keitai Wiki - "dedicated to cataloging games from Japanese Feature Phones"
The Cutting Room Floor - "dedicated to unearthing and researching unused and cut content from video games"
Hidden Palace - "dedicated to the preservation of video game development media (such as prototypes, hardware, source code, artwork, and more)"

Videogame wikis (specific)
Terraria Wiki
Sega Retro Wiki
Hard Drop Wiki - Tetris
Boktai
Pop'n Music Database

Very general wikis
Wikipedia
 Contropedia, Notabilia, Hatnote, Wiki Atlas, Welten 3d https://www.thewikigame.com/
Citizendium

Somewhat general wikis
Stanford encyclopedia of philosophy
 Visualizing SEP
Proof Wiki
Wikidoc

Unsorted specific wikis
Fanlore Wiki
Beta Wiki

AoPS wiki
Prime Wiki - prime numbers
Tricki - Math problem solving techniques
Math communication wiki

Openham - "an open repository of all things amateur radio"
SCP Wiki - "a collaborative speculative fiction website about the SCP Foundation, a secretive organization that contains anomalous or supernatural items and entities"
Pluralpedia
Camera Wiki
Media Wiki - Open source software for making wikis
sona pona

Web Revival Wikis
Melonland Wiki - "This is a wiki for all sorts of cool stuff, like tips for making sites n stuff"
Nekoweb Wiki
Penny's Pages - (maybe abandoned)