I have been working on plugins (which are programmed in Java) for Spigot which can be used by server owners to customize their server. My most sophisticated plugin implements a whole survey system which server owners can use to get feedback from their players. My most popular plugin features over 4.000+ downloads and is being used on 200+ unique servers.
Maybe a bit weird to stick it on here, but this site offers quite some functionalities. It features a portfolio. Items can be added on the fly, just by logging in and creating a new item. Items can also be edited and deleted. This site also features a file upload system for images and videos. The files get an unique id (uuid). When uploaded, the original name, uploader, and the time of upload is being put into a database.