I developed several libraries, which I use for own projects, but also developed with others in mind.
Jackson Bukkit is a library using jackson to serialize spigot and paper objects.
SADU is my SQL library, which I use to avoid a lot of boilerplate code, when sending and reading requests from a database. It was also written with novice developers in mind, making it easier for them to work and avoid common pitfalls.
Universalis Java is a library for requesting crowdsourced market board data of the game Final Fantasy XIV from Universalis. It also contains a websocket implementation, allowing to subscribe to receive realtime updates from universalis.