For those who want to be educated about widgets!
The widget is just a plastic container with a small hole in it. After filling, liquid nitrogen is dropped into the can before sealing with the end. The nitrogen vaporises and expands in volume after the can is sealed, forcing the gas into the widget's hollow interior through a tiny hole. Any remaining nitrogen dissolves in the beer.
When the can is opened, the pressure in the can quickly drops, causing the pressurised gas and beer inside the widget to jet out from the hole. This agitation on the surrounding beer causes a chain reaction of bubble formation throughout the beer. The result, when the can is then poured out, is a surging mixture in the glass of very small gas bubbles and liquid. Nitrogen is used as it generates much smaller bubbles, so a better head, than the carbon dioxide used to pressurise the cans normally.
Originally this widget was a chunky plastic device fitted in the bottom of the cans. Subsequent development (mostly by other brewers trying to bypass the patent held by Guinness) has produced several other designs including widgets that float on the surface of the beer. Many are now made from aluminium, especially for the American market, so the whole can is made from the same material and therefore recyclable.