The prime purpose of the KatMouse utility is to enhance the functionality of mice with a scroll wheel, offering 'universal' scrolling: moving the mouse wheel will scroll the window directly beneath the mouse cursor (not the one with the keyboard focus, which is default on Windows OSes). IMO this is a major increase in the usefullness of the mouse wheel.
Another feature involves the wheel button. Since the wheel button is not consistently used in Windows, KatMouse can use it for a kind of task switching: with a click of the wheel button you can push a window to the buttom of the stack of windows that is your desktop, making a recovered window the active window.
Limitations: Does not work with Logitech Mouseware, freezes for a short time when minimizing windows if window animations are active, some application which will not fully cooperate with KatMouse (Office 2000). Not updated since 2007 and considered abandoned.