I was waiting for a copy process to run one day and had Terminal open in the background, so I started fiddling as some would. I discovered that the Command key trick extends to selecting (at least text) and drag-and-dropping as well.
[robg adds: This hint describes some of the other things you can with the Command key and objects in background windows. I tested today's tip and found that it worked as described as long as the background app (where you start dragging the text) was a Cocoa app. I could, for instance, hold Command and drag a selection in a background Terminal window, then Command-drag that text to either a Cocoa or Carbon app, and drop it. At no point would either of the background apps come to the foreground. I had no luck with a Carbon app (using BBEdit and Word) as the frontmost background app -- they both came to the foreground as soon as I Command-clicked within their windows.]

