A game is an enjoyable past-time with a set of (not necessarily rigid) rules.
Wii Fit is this.
At least some of the activities in Wii Fit are what you would consider "play". For that reason also, it is a game. Perhaps it is not entirely a game, but certainly much of it is.
A game I'm developing with some friends:
www.xnagg.com/zombieasteroids/publish.htm
It is largely a technical exercise but feedback is appreciated.