The mouse-wheel gives you great options to handle program-tabs in the system panel as a third mouse button. If middle-click on a program tab, a new instance of the same program is opened right next to the existing:
If you hover above a program tab and middle-click on the preview thumbnail instead, the same instance of the program will be closed:
This gives you great flexibility when it comes to handling system panel tabs.
Middle-click Options on Panel Icons in Windows 7
