Drag and drop is one of the many wonders of graphical user interfaces, exponentially increasing productivity with an intuitive, common-sense approach that leaves you with a reaffirmed belief in the unquestionable brilliance of mankind. Exposé takes that to a whole new level: You can actually navigate Exposé while dragging a file/item, allowing you to easily locate and drop it into The Right Window.
Example: You’re surfin’ in Safari when you run across an image of Bob the Builder that would work perfectly as a title slide in your “Timmy’s First Halloween!” home movie project in iMovie. But iMovie is buried behind iCal, Mail, and that Word doc of your thesis you’ve had open for five years designed to remind you that “one day…[you’ll] get it to one day.” What are your options? Well, you could save the image to the desktop, pop into iMovie, then import it that way. Or save it to iPhoto, and use the Media Browser to locate and load it in. Both options require extraneous and tedious steps, as well as an extra file you’ll almost certainly never need again.
Instead, you could do this: Grab the image and, without releasing the mouse button, load Exposé (F9 is the default). Drag the item over the iMovie window and wait a few seconds (or hit spacebar) — then pop the image into one of iMovie’s media boxes, where it will be automatically imported into your project. The same kind of thing works for all sorts of items: Dragging files between Finder windows, text from one app to another, URLs across browsers, etc.
Update: Some readers made a great point, something I should’ve originally mentioned: Instead of having to use a second hand to hit the F9 key, just set an Exposé hotcorner and perform the drag-and-drop with nothing more than a swift motion of the mouse. I have my bottom-left corner set up for Exposé’s All Windows option, and it’s great for drag-and-drop scenarios. (Thanks Chadster and Rolphus.)
Reference : TUAW