DAY 19 - MEDIA WIKI
https://www.mediawiki.org/wiki/MediaWiki
It's hard to imagine the internet without Wikipedia. I use it and Wikimedia commons several times a month, but the wiki software is more than just Wikipedia. Pretty much every fandom knowledge site I've been on uses media wiki software or something based on it. It allows the creation of crowd-sourced comprehensive knowledge bases in a way that plain websites can't. Lately I've been working on fanlore https://fanlore.org/wiki/Main_Page wiki.