Editor

From Styleguide

editor

A piece of software that allows developers or players to more easily create their own personalized game content.

Typically, an editor provides tools that allow for the creation of new game levels, although the term also can apply to software used to modify other game elements such as gameplay variable values and game art assets.

Example: The Half-Life 2 editor provides powerful, yet easy-to-use tools for fans to create their own game scenarios.

Also see: mod.