The Wii Fit is a great way to get the family moving.  It features about 40 or so games, with different difficulties.  It covers 4 catagories; balance, strength training, aerobics, and yoga.  Some of the games can get quite addicting; I spent quite a while trying to beat friends' records on Hula Hoops!  The soccer game is another favorite as well.  It can be quite the party game if your friends aren't the shy types.