Carbon Config is a Config Library first Developed for IC2Classic and now made a Open Source implementation.

The idea behind Carbon Config is to have more customizable, simpler, more stable, cleaner and feature rich implementation of a config.

Carbon Config is built out of 2 Modules.

The GUI supports both CarbonConfigs and Forge Configs right out of the gate.

A detailed Feature list can be found here

To list a few of its features:

Compounds:
Data can be visualized via Compounds instead of raw Strings.
Suggestions:
Suggestions can be provided to the user in form of "default values" to make the user experience much more smooth.
Multiplayer Support:
You can edit "Any" Server sided config too, not just "server only configs"
Client to Server Configs:
Players can have their own settings in a Multiplayer instance and they can be easily distinguished.
Late Loading Configs:
If the config loads early but entries are "registered" late, that isn't a problem since the configs get parsed as they are registered.
Registry Visualization:
If it is Items/Block/Fluids/Potions/Enchantments/Colors get all visualized specially in the GUI.
Background Skin System: If you don't like the current background? Simply change it to one of the curated choices provided. There should be something for everyone!
Minecraft gamerule support: You can edit game rules of your current world/server or existing Worlds without having to open them. Which finally gives a use to the fabric version