It's all about value-based marketing; you have to provide your customer something helpful for free (e.g., knowledge, advice, an e-book, etc.), and once they regard you as a resource (read: a friend), they'll be a warm lead when you ask them to buy from you.

Dropbox, for example, provides its program for free, which is really handy and provides a large amount of storage. When you hit that storage limit, most customers are willing to pay for the premium subscription because Dropbox has earned such a stellar reputation. Many users would not have downloaded Dropbox if it demanded money up front.