Visual Memory Unit (also known as a VMS in japan) -- Sega's most recent contribution to the art of gaming.

The VMU is essentially a memory card for the Dreamcast console, though with an LCD screen and buttons attatched. They can be used as portable extensions to console games. When inserted into slots in the Dreamcast controller, they act as a secret window which only that player can see.

Quite a nifty innovation. Sony's attempt to copy it, the PocketStation, has flopped.