Switché modifies the Command-Tab shortcut—which already lets you cycle through running applications—to instead show you all active windows for that program, Exposé-style. You can also assign two other pairs of shortcuts: one that will switch between windows of the same app, and one that will switch between all windows.
While you can currently cycle through your open applications with Exposé by activating the “All Windows” command and pressing Tab, Switché makes this process quicker and more natural by integrating it with an already commonly-used shortcut command.
However, such convenience does come at a bit of a price: Kapeli charges $8 for the app. You can, however, download it and give it a try first, to see if it suits your needs. Switché requires Mac OS X 10.6.