If you're a fan of keyboard shortcuts, like I am, you probably use Alt-Tab to cycle between open windows. That's a better method, but Windows uses such tiny thumbnails, I often find myself squinting to see if the selected window is the one I'm after--and sometimes I actually end up with the wrong one.
Windows Vista and 7 users can also press Win-Tab to engage Flip View, a kind of Rolodex-style window selector. This should be the best option, as it uses much larger, easier-to-view thumbnails (a term that really doesn't apply here). But for whatever reason, I don't like it.
No, for my money, the fastest and most effective way to cycle between open windows is by tapping Alt-Esc. Doing so instantly switches you to the next open window, then the next after that, and so on. There's no interim step, no thumbnail view to squint at or cycle through. Tap once and bam: the next window appears. Not the one you want? Tap again and bam: next window.
(If you're a total newbie when it comes to keyboard shortcuts, make sure you hold down the Alt key, then tap Esc again and again until you land on the window you want. Then you can release Alt.)
This works especially well if you have just two windows open; Alt-Esc functions like a toggle switch, instantly hopping back and forth between them.