This donation-ware application has been keeping me focused for the past year (or more). I’m not sure it will survive Lion's shift to full screen applications, but for now it runs on all of my non-server macs. Spirited Away simply hides all inactive application windows after a user specifiable amount of time.
I don’t tend to use it as much on my primary Mac since I have a huge amount of screen real estate.1 On my MacBook, it’s really nice to have background apps simply cleaned away while I work. I like to have the clutter clear from my window while I work.
I have a 27” iMac with a 30” secondary monitor. I know it’s grotesque but I love it. ↩