The new forums will be named Coin Return (based on the most recent vote)! You can check on the status and timeline of the transition to the new forums here.
The Guiding Principles and New Rules document is now in effect.
For people too lazy to click links, Guinness calls it a rocket widget. It is full of nitrogen and due to the balance of pressure between the nitrogen in the widget and the beer in the can, it stays sealed until the can is depressurized (opened). Draught stouts on tap are pressured by nitrogen gas, so the widget simulates the taste (texture?) of an on-tap draught stout with nitrogen bubbles.
Posts
http://en.wikipedia.org/wiki/Widget_(beer)
Widgets!
Short answer: it makes the beer taste more like it came from the tap.