Windows enables switching from one application window to another by pressing Alt+Tab. While that's very straightforward, it will step you through every top-level desktop window, annoying if you're only interested in windows belonging to the current application.
Easy Window Switcher is a very simple tool which supports an Alt+' (Alt+backtick) hotkey to switch you between the current application windows only.
The program doesn't require installation. It'll ask if you want to run it at startup, but otherwise it can just be run as required, without cluttering your PC with shortcuts, new system tray icons or anything else.
Easy Window Switcher isn't exactly feature-packed, then, but it won't drain your system's resources, either: our copy used barely 800KB RAM (private working set).
Windows can natively switch between the windows of an application if you hold down Ctrl as you left-click a taskbar button.
There are slight differences in some situations, though - Ctrl+click steps through Internet Explorer tabs, Alt+' doesn't - and if you prefer a keyboard-only approach, Easy Window Switcher might be better.