A co-worker was option-clicking on an OS 9 desktop, which hid the foreground application. I have been using Macs most of my life and I had no idea you could do that. Anyhow I tried it in OS X and sure enough it worked!
Is this common knowledge? I didn't see it posted.
[Editor's note: I'm not sure it's common knowledge, and I don't think it's been posted here before. It's also not just a Finder thing -- if you option-click any background application's window, the foreground application becomes hidden.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2002012610231278