On Windows, Alt-Tab navigates to all open windows, making it easier to find a open web page (remember, no Tabbed browsing on IE), or an open IM conversation.
With OS X I am out of luck. I need to Command-Tab to the application, and then use the Window menu to select the open window that I want. I can also Ctrl-Click on the application in the Dock and select an open Window there. Or I can also use F9 and click on the open Windows. But F9 does not list any minimized window.
There is a nice keyboard shortcut, Command-`, cycles through all open Finder windows. So I can always use Command-Tab to get to my application and then use Command-' to cycle through its windows.