Adds a deep underground realm with different mapgen that you can reach with obsidian portals.
Wilhelmines Living Nether
Adds various creatures to your Nether.
Add monsters and dragons to your nether world.
Hallelujah Mountains (AKA Cloudlands)
Giant rocks floating suspended in magnetic eddies. (skylands, islands, fantasy, giant trees)
Makes stone and desert stone see through, also works with nether mod
An explosive nuisance.
Nether biomes and contamination
Add more nether biomes