Terra

What It Is
Terra is a modern world generation modding platform, primarily for Minecraft. Terra allows complete customization of world generation with an advanced API, tightly integrated with a powerful configuration system.
https://terra.polydev.org/index.html
Useful Commands
Terra is actually used through other plugins, especially Multiverse Core.
Config Packs
Terra is a framework for generating Minecraft worlds.  In order to actually generate a world, you need a config pack.  By default, Terra comes with one called Overworld, but there's plenty of others available.  Currently installed on this server are:
Overworld
The default overworld configuration for Terra 6.0+, containing over 140 different biomes.
https://github.com/PolyhedralDev/TerraOverworldConfig
Aeropelago
A Terra config pack themed around floating islands.
https://github.com/astrsh/Aeropelago
Substratum
A Terra pack that creates a completely underground world with several unique caves to explore within.
https://github.com/DeathShadez/Substratum
Origen
Origen focuses on adding a new and more creative / diverse terrain generation, without using any new blocks or items. This means that it is compatible with vanilla installations.
https://github.com/Rearth/Origen