When I'm reading ebooks, I find it quite useful to blank out everything except the reader. On the Mac, I use Backdrop to achieve this. What this app does is blank out everything (wallpaper, desktop icons, etc) apart from any on-screen windows. Is there a program that does this for Ubuntu?
Dimming non-active Windows with Compiz
In case we run Compiz (by enabling Visual Effects in System->Preferences->Appearance) we have a nice feature that lets you dim anything but the active windows to any desired value.
With the following packages installed (default from >= 11.04):
you will have access to control Compiz settings by running System -> Preferences -> CompizConfig Settings Manager.
Choose Accessibility: ADD Helper
Open Tab "Misc.Options:
and push the slider for brightness to the desired value (here depicted for 20).