Flexible level editor

A new Minecraft world editor and converter that supports all versions since Java 1.12 and Bedrock 1.7.

It's a Doom editor

Thunder Engine – Cross-platform 2D and 3D game engine with module architecture

A Tool to Help the Creation of Warcraft III Map

Map editor for pokeemerald, pokefirered, and pokeruby

A Python library for reading and writing the Minecraft save formats. See Amulet for the actual editor.

A map and tileset editor for pokecrystal, pokered, and projects based on them. Written in C++ with FLTK.

Online collaborative map editor for role-playing games.

Open source toolkit for Mafia games. (Mafia II, Mafia III, Mafia DE)

2D platformer engine using LÖVE and Lua

Tile / Sprite / Map Editor

A BYOND language smartness provider, map renderer, and more.

Kirby's Dream Course / Kirby Bowl level editor (C++ / Qt)

A DOOM and Duke Nukem 3D style game with ray-casting, featuring a level editor and multiplayer from scratch in C

XYZ Maps is an open-source map editor written in TypeScript.

Open source map editor in Unity, for Rust.

N-Space: a mobile app for building 3D interactive worlds

Kirby's Adventure Level Editor (C++ / Qt)

CAMM (Crystal Alien Map Maker) open source level/map editor for CrystAlien Conflict.