Say you're in the Finder looking at a file and you want to open it with an application that's already running but which doesn't own that particular document. How? Switch to that app and choose File > Open? Too many steps. Choose Open With from the file's contextual menu? Takes too long, and the app might not be listed. Drag the file to the Dock and drop it onto the app's icon? The icon might be hard to find; worse, you might miss.
In Leopard there's a new solution: use the Command-Tab switcher. Yes, the Command-Tab switcher accepts drag-and-drop! The gesture required is a bit tricky. Start dragging the file in the Finder: move the file, but don't let up on the mouse button. With your other hand, press Command-Tab to summon the switcher, and don't let up on the Command key. Drag the file onto the application's icon in the switcher and let go of the mouse. (Now you can let go of the Command key too.) Extra tip: If you switch to the app beforehand, its icon in the Command-Tab switcher will be easy to find; it will be first (or second).
From Matt Neuburg