Mapping Extra Buttons

A lot of modern computer mouses have more than two buttons. They also have a clickable wheel and extra buttons.

I use a Logitech Wireless Trackball M570 with has two customizable extra button. At the moment I mapped one

to the ENTER key and the other to navigate back.

How do you map extra buttons on your mouse?

If your keyboard has mappable extra buttons, what function do you give them?