Empire II: The Art of War Map and Scenario Editor.
Empire: Wargame of the Century Map Editor (incorporated into the game program).
The Elder Scrolls Construction Set construction set for The Elder Scrolls III: Morrowind and Oblivion.
Dynamite Jack editor for Dynamite Jack.
Dragon Dragon Age Toolset for Dragon Age Origins.
Doom Builder, a level editor with an ACS scripting editor for Doom and Doom II and source ports such as ZDoom.
DFXMED Mission Editor for Delta Force Xtreme.
DFBHDMED Mission Editor for Delta Force: Black Hawk Down.
DromEd for the Dark Engine used by Thief: The Dark Project and Thief 2: The Metal Age.
Commando Level Editor for Command & Conquer: Renegade.
Creation Kit construction set for The Elder Scrolls V: Skyrim, based on the previous The Elder Scrolls Construction Set.
Construction mode for Mega Man Powered Up.
Combat Mission: Beyond Overlord Mission Editor.
ChromEd Editor for the Chrome Engine used by Chrome and Call of Juarez.
Build, for Build engine based games such as Duke Nukem 3D, Shadow Warrior or Blood.
Build 'n Race lets you build your own race.
Boom Blox has a create mode where you can make your own levels and puzzles.
Blast Works has a level, ship, shape, and bullet editor.
Aurora Toolset for the Aurora Engine used by Neverwinter Nights, Neverwinter Nights: Hordes of the Underdark, Neverwinter Nights: Shadows of Undrentide and The Witcher.
ArmA and ArmA 2 have an in-game mission editor similar to their predecessor Operation Flashpoint.
Anvil, a "physics model" editor used for game variables in Marathon 2: Durandal and Marathon Infinity.
ScenMaker Scenario Editor for Airborne Assault: Red Devils Over Arnhem.
Age of Wonders II: The Wizard's Throne Scenario Editor.
The Age of Empires series Scenario Editors.
Advance Wars Design Room for Advance Wars and Advance Wars 2: Black Hole Rising on the GBA, and for Advance Wars: Dual Strike and Advance Wars: Days of Ruin on the Nintendo DS.
Act of War: Direct Action and Act of War: High Treason Map Editor.
A-Train HX Map Editor on Xbox 360 (In-Game).
Just to name a few, the best or most fun are highlighted in Red.