Make your own modules and upload to Steam Workshop for Shadowrun Returns.

Includes GECK a level editor for creating levels for Fallout 3 and Fallout:New Vegas.

Has a built in editor for making levels for Monaco which can be uploaded to the Steam Workshop.

This game has a level editor for making new maps for this RTS and uploading to the Steam Workshop.

Not really a game, but a tool for making JRPGs. This is a great tool for anyone wanting to get started in making rpgs.

This game allows access to an editor for making platformer levels for Super Meat Boy. Great for practicing platformer level design.

Legend of Grimrock has a built in simple to use level editor that use lua for scripting.

This game allows the player access to the GUTS tool which can be used mod/design levels for Torchlight II.

Portal 2 has an in-game level editor for making quick puzzles that can easily be uploaded to Steam Workshop. Great for level puzzle design practice!