The Balance Board is a step pad which will allow realtime detection of steps, twists, push ups and a whole variety of other activities related to the shift of balance across the pads.
Accompanied with the peripheral will be Wii software titled Wii Fit which includes a ranged of minigames designed to be used in a accompaniment with the balance board. The software includes a range of minigames, of which a soccer game and twisting game were shown at the E3 conference. Players were also shown in premade footage doing press ups and yoga type activities on the Balance Board.
The software also allows for gamers to weigh themselves and be shown a full breakdown in graphical form of their Body Mass Index - which is then put into a graph and can be monitored over a period of time similarly to the fitness tests in Wii Sports.
It was also hinted that the Balance Board will be able to be used in other software titles: snowboarding and Tony Hawk's games spring to mind.
Here is a video of the announcement: