Previously mentioned Alt+Tab replacement VistaSwitcher is one of the coolest new utilities we've featured in a while, and today it's updated with out-of-the-box support for application-specific Alt+` window switching.
To switch between any open Windows, the standard Alt+Tab is still your go-to shortcut. If, however, you want to switch only between individual windows of a single active application, you can now hit Alt+` (the backtick key next to the 1 key on the top of the keyboard). So, for example, if you've got multiple Firefox windows, an Explorer window and a Word document open, but Firefox is the active app, you can switch only between Firefox windows by hitting Alt+`. This feature is actually lifted from OS X, but it's a really nice shortcut to have in Windows, too. (Too bad it's not one of the awesome new Windows 7 shortcuts.)