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. Doodim's product site offers a simple animated screenshot demo to help you see what really goes on, and users can even take the dimming effect all the way to black, so no nothing else is visible, save for the app you're working in. It's a slick idea and is done well, and toggling the dimming effect is simply handled from the menubar (the menubar, if you're wondering, is not dimmed). The site lists one known catch so far, which I personally haven't run into while playing with this: apparently, the dim effect can fail sometimes while switching between apps. Bouncing from Firefox to Mail, Adium, NetNewsWire, Safari and iTunes hasn't produced any issues for me yet though.
Doodim is provided free from La Chose Interactive, and I'm not sure if it's 10.4-only or not.