I've recently been thinking a lot about the freemium business model. For those unfamiliar with the freemium business model, it was first articulated by venture capitalist Fred Wilson in 2006:
"Give your service away for free, possibly ad supported but maybe not, acquire a lot of customers very efficiently through word of mouth, referral networks, organic search marketing, etc., then offer premium priced value added services or an enhanced version of your service to your customer base."