MAST - Modular Asset Staging Tool - Asset Store
James Hicks

MAST is an editor extension for Unity, primarily intended for staging 3D modular assets. It is very useful for making scenes and levels that are based off of a grid. Instead of having to drag a prefab into the inspector, then manually move, rotate and flip it, MAST provides a simple interface that lets you do all the above before ever placing the item in your scene.