In Mac OS X 10.5, you could combine the use of Exposé's Application Windows mode and the Command-Tab application switcher by first activating Exposé and then pressing Command-Tab. When you selected another application and released Command-Tab, that application would come to the foreground in Exposé mode, revealing all its windows. Alternatively, you could release just the Command key, and cycle through all your applications (while still in Exposé mode) by just pressing the Tab key.
While this worked well, it required you to activate Exposé before pressing Command-Tab. In Snow Leopard, this process has been greatly improved. While the old method still works as before, you can now activate Exposé's Application Windows mode after pressing Command-Tab. Don't try to do this with the Exposé key on your keyboard, though, because that won't work.
Instead, keep the Command key pressed, release Tab, and then press either the Up or Down arrow. This will switch you into Exposé's Application Windows mode for the currently-select application in the Command-Tab app switcher. At this point, you can release the Command key, too, leaving no keys pressed.
From here, press Tab (and Shift-Tab) to cycle forwards and backwards between applications (while remaining in Exposé's Application Windows mode). When you see the window you'd like to activate, press one of the arrow keys to select it--the selected window will gain a blue outline (or you can use the mouse/trackpad, of course). To activate the chosen application with the selected window, just press Return to exit Exposé mode, and the selected window will come to the foreground.
Another nifty Exposé trick in 10.6--and this works both in the above example, and when using Exposé's normal Application Windows mode--is the ability to Quick Look a window with Exposé active.
Make sure the window is selected (shows a blue highlight) by pressing an arrow key first or hovering over it with your mouse, then press the Space Bar. The selected window will zoom in, just as with Quick Look in Finder, but you'll remain in Exposé mode.
This combined mode can be very useful if you keep tons of windows open, and/or if many of your apps use smaller windows that really aren't distinguishable in Exposé mode. (Notice, too, that Exposé now displays the title for every window; in 10.5, you only saw titles when you moved your mouse over one of the windows.)
Thanks to Mac OS X Hints readers Benji Coleman-Levy and chumsdock for the observations on the new abilities of Exposé in Snow Leopard.