Exercise, or the (virtual) puppy gets it!
The research study (pdf) used virtual fish in a common fishbowl and standard pedometers whose displays were read using OCR. There are many obvious potential improvements, but it seems like an excellent start at a way to motivate people to achieve any number of goals.
a persuasive data visualization that maps a player’s daily foot step count to the growth & activity of an animated virtual character, a fish in a fish tank. the "Fish’n’Steps" application links the size of a fish as the step count, while success in reaching a participant’s daily goal affects its facial expression (e.g. happy, angry or sad). baby fish appear once the upper appearance level is reached.(ht: information aesthetics )
some fish tanks also include other players’ fish, to create an environment of both cooperation & competition. in a 14-week study with 19 participants, the game served as a catalyst for promoting exercise & for improving game players’ attitudes towards physical activity.
UPDATE: In the comments, Dani mentions vMigo, a virtual dog product where the game controller includes a pedometer and you get "pet points" for taking your virtual dog on walks with you. What a great feature!