The Unity documentation isn't very clear on how to setup scripts to add features to itself. I found it rather confusing. This is mostly because the biggest step, where to put the script file to make it work, isn't explained very well. I aim to help some of you out with this post, having got it working this afternoon.
When setup right, having the Editor scripts folder and hooking code into the Unity Editor interface is very powerful. Such a setup allows one to write custom tools. In short, everything in the Unity graphical user interface(GUI) that you use to make your game project is written in Unity.
A quick disclaimer: I am no expert on Unity. I've got less then a year's experience using the tool, so don't take my words as law. If you found this page in a google search, I'm probably about as experienced as you.
Anyway, enough rambling. On with the useful info (after the break):