Found out, there is a plenty of project extending YaCy on GitHub.
This list may be inspiring:
https://github.com/cscm/YacY-Munin-plugin
YaCy Monitoring Plugin for Munin
https://github.com/frankenstein91/sci-hub2Yacy
Make Yacy Crawl Sci-Hub
https://github.com/frankenstein91/yacy2xmpp
yacy to xmmp (jabber) bot
https://github.com/Pipazoul/yacy-search-frontend
A svelte static frontend to search into yacy documents
https://github.com/Stepanov-Sergey/yacy_synonyms – yacy_synonyms
Dictionaries of synonyms for different languages
browser integration
https://github.com/JeremyRand/YaCyIndexerGreasemonkey
YaCy indexer script for Greasemonkey to index browsed pages
https://github.com/rob-luke/safari-to-yacy
apple’s safari to yacy
https://github.com/blahgeek/firefox2yacy
send firefox history to yacy
https://github.com/rickoooooo/IndexThis
An Android app that allows you to index a URL to a YaCy instance.
do you know some other projects?