BananaSoft has created a freeware application called FocusLayer, which allows you to dim everything but the active window. The amount of dimming is adjustable, and the application also comes with an optional menu extra. I've not seen someone come up with this before, and it seems visually quite useful. Check it out!
[robg adds: This is a very unique and useful application. In limited testing this morning, the only potential downside I see is that it takes a bit of CPU power -- anywhere from 5% to 25% of my G4/733, depending on what I was switching to or from. But it's certainly interesting enough to merit a try out -- you can also choose between dimming non-active windows (the default) or non-active applications, and whether you want to use a dock icon, menu bar icon, or both to control the program.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030427021817295