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
General
Wikipedia - "The Free Encyclopedia"
Contropedia - Wikipedia in-article controversial edits viewer
Notabilia
Hatnote - "Listen to Wikipedia"
https://wiki-atlas.org/
Stanford encyclopedia of philosophy
Visualizing SEP - A visualization of the above website
Media Wiki - Open source software for making wikis
Fanlore Wiki - "a wiki about fanworks and fan communities"
Beta Wiki - "An open encyclopedia of software history"
Yuri Anthology Wiki
Proof Wiki
Prime Wiki - prime numbers
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
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)