Unless you have kids, you may have not heard of Club Penguin? It's basicall an online community for kids which has built in mini games. You create your character and then you can adopt a 'Puffle', chat to other games and buy stuff for your character and your Igloo.
The thing is, it's totally free to play....but, just as your kid has got hooked on playing it, and they then want to play the extra levels of the games, or adopt the cooler 'puffles' ...or buy nicer items for your Igloo, then you have to sign up for a monthly subscription. You can either pay monthly (5 bucks) or you can pay for 3 months, 6 months etc up front for a cheaper rate.
Of course, I buckled from the pressure of my 6 year old daughter so I'm now paying for this sh1t on a monthly basis lol. Anyway, it's a great example of giving value up front for free, and then charging on the back end. It was so successful, it was recently bought out by Disney for quite a bit of money ;-)
Think about how you can apply this strategy to your own business...nice