Planetoids - Generates a world full of planets:
Version: v1.4.2 Release

Planetoids, by default, creates a new world (called "Planetoids", if you fail to make a settings file) that uses my custom generator class to create a world full of "planets", which are spherical objects of varying size that have a "shell" of one block type, and a "core" of another. Planetoids can also be configured to work with multi-world management plugins, serving as a chunk generator for worlds that you create.

I got the idea from the Planetoids Map Generator, but I have no affiliation with the author of that tool.

Features:

Configurable core and shell blocks, with percentage chance
Optional floor, with configurable block type and height
Optional single-block bedrock floor below that (to hold liquid floors)
Can lock it to nighttime only
Can disable weather, monsters, animals
Reads world height from server.properties. Only seems to work up to 256.

Download Planetoids
Installation and Troubleshooting

A default config is created if you don't use the one from the zip, inside plugins/Planetoids. If you want to customize it before the spawn area generates (highly recommended), make sure you edit the one from the zip and put it in the right place. If you're going to use Planetoids with a multi world management plugin, I suggest you set 'autocreateworld' to FALSE, or Planetoids will make a world for itself automatically.

Notes (Read this before asking questions please!)
Screenshots
Changelog/Planned Features
Source Code on Github