Apple introduced Exposé with Mac OS X 10.4 "Tiger" in April of 2005, however, many Mac users either don't realize that it exists, or don't know how to utilize it to the full extent. Exposé helps you deal with the constant clutter of overlapping windows and the need to drag and drop items from one window to another by quickly getting things out of your way.
Before getting started, make sure you have set up the keyboard shortcuts to your liking - open System Preferences and choose "Dashboard & Exposé." On a laptop Mac where many of the function keys are already mapped to things like screen or keyboard brightness, it helps to double up Exposé on the two remaining keys, F11 and F12 (screenshot). On our setup, pressing F11 shows all windows, while ⌥ (option) F11 shows only windows in the current application. F12 reveals the desktop, and ⌥F12 shows Dashboard (Hint: to set the keyboard shortcut to use ⌥, hold it down while clicking on the shortcut menu.) It's also possible to set screen corner shortcuts, but many people find these confusing since they are often bumped by accident.
Aside from simply revealing your myriad windows and letting you quickly bring the desired one to the front, Exposé really shines when combined with drag and drop. Drag and drop has been a hallmark of the Macintosh experience since the beginning, but it can be difficult to use since the source and target windows are often buried under multiple layers. Exposé can help. Assuming your keyboard shortcuts are set up as indicated above, press F12 to reveal the desktop, grab a file, and start dragging it. While still dragging, press F11 to reveal windows in all non-hidden applications. Drag the file over the desired window and either press F11 again to immediately bring the window to the front, or pause for a few seconds and Exposé will bring it to the front automatically. Drop the file. You have now completed a drag and drop without moving any windows!
Variations of this can also be used to drag things between programs without involving the desktop. The key to the whole experience is to not let go of your dragged item until you've tried out various Exposé shortcuts.