This project is unmaintained, but feel free to fork it.
This is a plugin for building level editors for Unity games. It lends a few design choices from the Unity editor itself, but has some more tricks up its sleeve. I am developing this in tandem with my Deus Ex-based game project The Vongott Chronicles
If for whatever reason you'd want to include a level editor in your game, this is a good place to start. OpenEd at its default configuration is very reminiscent of the Unity editor, but because it's using OpenGUI, you can make it look and behave any way you want.
In the wiki
Faded grid
Full nested object serialization
Prefab browser
File browser
Custom inspectors
Asset linking via either compiled resource or custom zip-based asset bundle
MIT