If you use the Mac OS X Spaces feature, you can get an overview of all your workspaces by pressing the F8 key. However, seeing the individual windows within each space is difficult in this mode, because they can be tiny – but there’s a solution to this problem.
After pressing F8 to show all workspaces, hover your cursor over one of the individual windows and press the spacebar to invoke Quick Look. Doing this will give you a full-size preview of the window. You can then click on the window to make it active.
You can also use Exposé within the Spaces overview: press F8 for the overview, then F9 to show all windows. If you press the spacebar once while hovering over a window and then move your mouse over another window, you’ll get a Quick Look preview of that one, too.