Click on a portion of the image and drag the mouse around; a rectangle is drawn that shows the region enclosed by the starting position and the current position. I've seen this used frequenly for Java front-ends to GIS systems. The user selects a region of the current map using the rubberbanded rectangle. When the user releases the mouse button, the coordinates of the rectangle are sent back to the GIS system, which draws a new map based on the extents of the rectangle that the user selected. The rubberbanding technique is also useful in "paintbrush"-type programs that allow the user to drag to resize their object.