I also find DimScreen (DimScreenMod) to be quite useful when using my computer after dark, it really helps my eyes. That along with Blank Your Monitor extension for Firefox.

I am bumping this thread because it really would be great if the overlay could actually always be on top of everything!

However I'll continue to use it as is, because it still works the best out of all the tools I have tried.

Yes, I would like to have the option to hide the tray icon as well.

Disabling multiple instances shouldn't be permanent but just an option so those who want multiple screen margins can run multiple instances.

@johnye_pt: If your DesktopCoral has a background you can just right click on it and select options (rather than launching the application again) - see mine. Of course having a background probably doesn't fit into your setup.

Hmm... I just found someone else requesting this:
http://www.windows7taskforce.com/view/2251