Darken apps in the background with Doodim:
"For those times when you need complete, uninterrupted concentration on a particular app you're working in, there is Doodim: a simple menubar utility that creates an Exposé-like dimming effect on the desktop and all background app windows."
(Via The Unofficial Apple Weblog (TUAW).)
Here is something for Jon Udell to try for his distraction-free desktop. I haven't tried it myself yet, I'll update here once I have. It might have the same distraction effect that Spirited away has.