Bukkit topic
Bukkit is a Minecraft server modification software and API, and while the original project itself is stale, forks like PaperMC and Spigot are actively maintained, and most plugins and servers use some variant of Bukkit.
TaskChain
TaskChain Control Flow framework. Helps facilitate running tasks on an application's "Main Thread" (such as a game), and tasks off the main (async).
Akarin
Akarin is a powerful server software from the 'new dimension'
LuckPerms
A permissions plugin for Minecraft servers.
MC-Remapper
Deobfuscator for Minecraft by mapping json
MCAntiMalware
Anti-Malware for minecraft
Paper
The most widely used, high performance Minecraft server that aims to fix gameplay and mechanics inconsistencies
Glowstone
A fast, customizable and compatible open source server for Minecraft: Java Edition
Purpur
Purpur is a drop-in replacement for Paper servers designed for configurability, and new fun and exciting gameplay features.
Mohist
Minecraft Forge Hybrid server implementing the Spigot/Bukkit API, formerly known as Thermos/Cauldron/MCPC+
Tuinity
Minecraft server software fork of Paper to improve performance without behavioural changes.