Windows only: Tiny utility ActivateByNum mimics the Windows 7 shortcut key combination for quickly switching between open windows—you can simply use Win+n to choose open windows based on their position on the taskbar.
If you are using the Windows 7 release candidate already, this ability is built right in—simply use Win+1 for the first taskbar button, Win+2 for the second, and so on, to switch between open windows based on their position in the taskbar. If there is more than one instance of your application running, using the shortcut key again will switch to the next open window in the series, similar to Mac's Cmd+` shortcut key combination. Once you've downloaded and launched ActivateByNum, you'll have the same feature in XP or Vista.
If this seems familiar, it's because you could use the same shortcuts in Windows Vista to launch applications from the Quick Launch bar, but ActivateByNum overrides those keys to provide the Windows 7 behaviour.
ActivateByNum [Google Code]