Sony have recently made their level editor fully open source!
Here is a link to the github
The LevelEditor is a software tool used to design levels (maps, campaigns, and virtual worlds) for a video game. A level editor is used by a game designer.
The standalone LevelEditor is a fully functional modern level editor. Using the LevelEditor, you can design a game world for any video game engine. You can create and lay out terrain, place static game objects in the world (such as rocks, plants, street lights, or buildings), place light sources for game objects, and place Linears for dynamic game objects.
The LevelEditor leverages the Authoring Tools Framework and C# for user interface and data management, and leverages the power of C++ and Microsoft® DirectX® 11 for 3D rendering.
Originally built from the Authoring Tools Framework, the LevelEditor offers a WYSIWYG interface and a variety of features that support asset management, game object creation, scene layout, and cross-team development. The LevelEditor can also be customized and extended by creating plug-ins. For more about the Level Editor's capabilities, see LevelEditor Features & Benefits.
The following LevelEditor features help you construct game levels efficiently and collaboratively:
Work with a variety of file formats
Associate assets with game objects
Position, rotate, scale, and snap game objects precisely
Edit game object properties
Show or hide groups of game objects to unclutter the view as you work
Construct Linears (lines and curves)