Cross communication between forge & spigot has never been more simple!
This plugin library allows you to easily communicate with forge mods, allowing you to make client-side mods with exclusive server features. Such as more detailed game HUDs etc. all the while supporting and handling it server side without much of a hassle.
More information can be found on the wiki page linked in here.
It is required for this plugin to be running if you want to use it's API.