I understand I can create an Editor folder in my Assets folder and put editor scripts there, they will be available to my project.
Is there any way (a global folder or setting) to make scripts available to ALL my projects? Meaning, the handy stuff like 'copy transform', I use in every project, I want to have one copy of that, and any project I open will have that available.
Probably the most convenient way to achieve this is using packages. Simply create a package of your editor scripts, and put it to a location that's easy for you to look up. Then, when you create a new project, simply import your editor scripts package and you should be all fine. This is under Assets / Import package, Export package ... docs are here, in How do I reuse assets between projects?
If you have to make changes, you can also update those packages (it's a manual step, though, which you'd have to do for each project).
If you're on Mac OS X, you might try symbolic links. I'm not sure how well those work with Unity, though, but it would be worth a try (there's also "hard links" which might work better ... actually, I think the OS should treat those exactly as if the file/folder was actually there).
EDIT: Actually this should also work under Windows ... see for example this thread: how to create a symbolic link under Win2K, WinXP, and above?
While setting up a link is one extra step in setting up the project you have the nice benefit that you don't have to worry about updating many projects whenever you have a change.