New Biomes
Arkatron

If you like underground biomes, this thing is for you.

Unstable Caves
M BLOC

Some stones may cave in if punched, making underground expeditions more risky.

Beautiful Boulders
regulus

Procedurally Generated Rocks with (pretty) Accurate Collision Boxes

Nyancat's heaven (reworked)
Hybrid Dog

Adds a nyanland in the sky

Tac Nayn
GreenXenith

Adds Nyan Cat's evil twin, Tac Nayn (and a gray-scale rainbow).

Sticks Stones
MikeRedwood

Makes Sticks from leaves and scatters small stones

Purple ore
X-DE

Adds new purple ore to floating islands and caves from height 200 to 31000.

treasures
ademant

Treasure nodes with useful items

golden lava
GigaByte

add golden lava

Trucraft
mt-mods

An alternative crafting system done entirely in the world.

Obdy
Sires

Adds some pretty OP tools :)

Spawn redo
cathaya7d4

Drop-in replacement for default/spawn for more third-party biomes

Underground Survival Lighting
Arkatron

If you like underground light in survival, use this mod, it can help that.

Mud
Blocky Player

Adds stylish mud blocks and even blocks you can walk through. Setup traps, secret rooms and much more!

Silver
Jackknife

adds silver ore and tools

Too many ores
TeufloV

This adds 53 new ores. But most of them are useless.