This script inverts the editor’s hierarchy selection.
DOWNLOAD EDITOR SCRIPT
HOW TO USE IT
You need to place the script InvertSelection.cs in a folder called Editor in your project’s Assets folder for it to work properly.
After you import it, go to Selection > Invert to use it.
I personally use it when setting up LODs, I save a lot of time using this. For example, if I need to apply the same Material to a lot of different objects which has LOD Groups:
This script was taken from Unity Wiki.