When you open a .dmg file, a disk image is mounted. You are then generally supposed to copy the contents of that disk image to your hard drive (to your Desktop, your Applications folder, or wherever). But what if you want to copy the whole disk image, including all its contents, as a folder? Hold the Option key, and drag the "proxy icon" in the title bar of the disk image window to the destination in the Finder.

From Matt Neuburg