What device, patented by Guinness, is found in some beer containers? What does it do?

Great question! The "widget" is a plastic ball smaller than a ping pong ball. The widget has a few holes so that beer enters it while the can is unopened. Due to the nitrogen pressure at the top of the can, the change in pressure by opening the can will cause the beer to rush out of the widget. This exit causes turbulence and bubbles to form. The combination of the turbulence and the nitrogen leads to smaller bubbles making the beer head more creamy...and more like a draught beer.

https://en.wikipedia.org/wiki/Widget_(beer)#:~:text=A%20widget%20is%20a%20device,patented%20in%20Ireland%20by%20Guinness.