If you have OS X 10.6, click and hold on the application icon to view all windows for that application. Or, just invoke Expose to view all windows by using a 4-finger swipe down on your trackpad or use the F3 key. You could also do a combination of both where you use Expose to view all windows, then click (no need to hold) on the application icons in the dock to view windows only for that particular application. When in Expose, you can also press the Spacebar as you hover over the windows to see a larger preview.