WorldEdit
sfan5

In-game world editor. Use it to repair griefing, or just create awesome buildings in seconds.

Schematic Editor
Wuzzy

Advanced tool for modders and advanced users to create and edit schematics.

Void - An empty game
Linuxdirk

The purpose of this game is to test mods in an absolutely “clean” environment with absolutely no “3rd-party” mods

Skyblock: Zero
zanderdev

Start from absolutely nothing in a skyblock world. Time is your resource.

Woolen Mesh Creator
entuland

Not only wool! Miniaturizes builds and allows rebuilding them too

LuaJIT Profiler
jwmhjwmh

Profile mods using LuaJIT's built-in profiler.

Find Biome
Wuzzy

Add commands to list and find biomes

Formspec Editor
Just_Visiting

A Realtime in-game formspec viewer/editor

Modgen
BuckarooBanzai

mapgen mod creator

Development Test
Luanti

Testing environment for engine or mod development.

Yourland Speak up
Sokomine

NPCs deliver speeches

Not actually the bare minimum
Sumi

Very little content so you can build off it. Totally permissive, you can relicense it or do whatever.

audio_lib
Zughy

Library to easily manage sounds (accessibility options!)

Imaging
entuland

An image / bumpmap importer mod

Cross-Section
Silver Sandstone

Cuts the world in half.

Essentials
SkyBuilder1717

Adds a lot of helpful commands for your server

boomstick
fluxionary

a tool for testing the behavior of tnt

Better Command Blocks
ThePython10110

Adds command blocks similar to those from a certain other voxel game.

BiomeDB
EmptyStar

Searchable biome database for robust biome selection

Node Touch Action
Neuromancer

Pass in a node name and a function, and every time a player touches that node, the function happens

Inspector
sofar

An in-game tool to inspect node parameters and metadata.

Debug
LMD

Debugging on steroids

Lua2Mts
Neuromancer

Allows converting .lua schematics files to .mts files.

ASCII Art Schematics
Warr1024

Manage Minetest schematics as ASCII art.

Lua Wand
Zemtzov7

Tool with programmable on_use

Perlin Explorer
Wuzzy

Developer tool to create previews of Perlin noises

Flow Inspector
Luke

An "inspect element" helper for flow GUIs

debuggery
fluxionary

debugging tools for server admins

QA-Block
bell07

A mods development / QA helper

Runtime Strictness
LMD

Disallows questionable usage of the Lua API & Lua itself to help catch errors

Luwua
LMD

:3

MT_QOL
beaver700nh

Improves in-game QOL by adding helpful commands

Texture Generator
LMD

Dynamically generated texture packs

futil
fluxionary

flux's utility mod

Ingame Editor for Yourland Speak up mod
Sokomine

Edit ingame what NPCs say without server restart

Cave Generator
Noordstar

A custom cave generator engine

K WorldEdit Command Builder GUI
ketwaroo

Alternate worldedit frontend to build and run commands that is agnostic of inventory.

Worldedit Big Schematics Handling
IRI

World Edit commands allowing to create and place large schematics, as well as maintain a command journal for undoing past actions.

Minetest Object IDs (API)
zeuner

Persistent object-IDs for developers

Too Many Aliases
ThePython10110

Adds aliases to every item/node in MineClone to make them match MineCraft so you can stop forgetting that mcl_mobitems:ender_pearl should be mcl_throwing:ender_pearl.