Guinness Stout in the big, ominous-looking black can, or the skinny new black bottle. It has a neat toy inside: the Guinness Floating Draught System, a plastic widget full of pressurized nitrogen, which gives the canned or bottled stout the same thick, white head that you'd get out of the tap.
Guinness Floating Draught System
Inside every can of Guinness Draught there is a little plastic ball which is filled with liquid nitrogen before the can is sealed. Once it is sealed, the nitrogen vaporizes and pressurizes the can. When you open the can, the pressure on the nitrogen is reduced, causing it to jet out of little holes in the ball. This aerates the Guinness, which gives it a better head when you pour it into a glass.
Other brands of canned stout now use similar devices, such as the Beamish "Floating Widget". Guinness appears to have been the first, though.
This is all very clever.
Update, October 2001: Guinness Draught is now available in bottles, which also contain a Floating Draught widget. The widget in the bottle is shaped like a torpedo, so as to fit through the bottle's neck during filling. Bottled Guinness Draught is meant to be drunk straight from the bottle, unlike the canned variety.