The Eco Button connects via USB and puts your PC into sleep mode, thus saving precious Watts and making Al Gore smile, despite the fact that he has become portly in a William Shatner kind of way. It will even wake the PC up, when it’s time to get back to work. The bundled software records the carbon units saved.
As Ubergizmo pointed it, it probably takes a lot of carbon units to build the button and then when it hits the trash one day, it will pollute anyway. Plus it looks like a Frisbee. This somewhat pointless green gadget will cost you $18.