Freemium

Freemium is a business model based on the idea that creating products and giving them away for free can create income by selling additional services on top of the main product.

See for more info:
 * http://blog.p2pfoundation.net/freemium-a-business-model-for-p2p/2008/12/08
 * http://www.slideshare.net/froberg/meet-freemium-presentation?type=powerpoint
 * http://www.freemium.eu/category/basics/