John Gruber, via GUIdebook:
…[Y]ou can use Command-Tab switching when you’re in the middle of a drag. So you can start dragging something in one app, then use Command-Tab to switch to another app, and then complete the drop in the new app. I don’t even know when this happened – it might have been like this on Mac OS X all along, but I don’t think I noticed until sometime during the 10.2 era. This also works with things like Exposé and Dashboard.
It sounds obvious, but doing something like that was completely unheard of on the old Mac OS.