Duolingo is a free website that allows users to learn a second language. It is set out like a game, offering a levelling system and awarding coins for good performance. The interface is extremely user friendly and the owl mascot is something that children in particular could relate to.
When considering the department for health brief, it would be an idea to make a game out of fitness - to make it fun and informative.
26/02/2013 UPDATE: Duolingo emails the user if they realise they haven't been online for a while. It is designed as a friendly reminder, encouraging users to play on, without being forceful and unpleasant. It gives the user an achievable goal, and I have to admit that I was tempted to play on, if I didn't have other things on. If I had free time I would definitely click on the link provided.