Archaeology
SkyBuilder1717

Adds an looting and API in archaeologing

Ancient Ice Stuff
Robot10&Scheletor

Adds a new powerful ore to the game

morebiomes
mzs.112000

A simple mod adding a few biomes to Minetest Game

Unstable Caves
M BLOC

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

Nyancat's heaven (reworked)
Hybrid Dog

Adds a nyanland in the sky

Kitty/Kitto Ores
nemokitty9

Nemokitty9's own ore pack. Includes Tools!

Fragments
Amaz

A mapgen with a goal, with things needed to advance to the next level.

Startest
Robot10&Scheletor

This mod adds several space stuff to the game.

Tac Nayn
GreenXenith

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

World Borders
argyle

Add impenetrable barriers to the sides and bottom of the world.

Beautiful Boulders
regulus

Procedurally Generated Rocks with (pretty) Accurate Collision Boxes

Zinc
alerikaisattera

Zinc metal, used for making brass

Purple ore
X-DE

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

golden lava
GigaByte

add golden lava

Obdy
Sires

Adds some pretty OP tools :)

Trucraft
mt-mods

An alternative crafting system done entirely in the world.

Spawn redo
cathaya7d4

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

Sun Flowers
Hybrid Dog

This mod generates flowers which close their heads at night.

Noriel Sylvire's Minerals Core Modpack
Noriel_Sylvire

An API for automatically registering recipes, nodes and ore generation.

More Coral
Nathan.S

Adds coral in more colors.

Fly-B-Gone!
Hugues Ross

Hides butterflies/fireflies

treasures
ademant

Treasure nodes with useful items

Underground Survival Lighting
Arkatron

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

Sticks Stones
MikeRedwood

Makes Sticks from leaves and scatters small stones

Dirt Tunneler
Neuromancer

Makes dirt deeper and you can tunnel through it just by walking through it. Press spacebar/shift to tunnel upward/downward with auto placed ladders.

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.