True Information Based Businesses

I am looking for a list of businesses that have all of the following charateristics: * They are based off of true viral marketing and by this I mean that the marketing is a part of the business' core product itself such as Hotmail. Or it requires people to have the same product to communicate together such as ICQ. I am not looking for businesses that do not contain these core values of viral marketing. * The business' core product is information based and requires no physical delivery of goods in order to turn a profit. * The business' core product must be subscription based OR pay-as-you-go. I am not looking for businesses that charge a one time fee for their products. * The business cannot be content based such as having content that is regurarly updated by its staff such as newsletter or news service sites. * The business model must be legal in the U.S. i.e. No online betting companies. My current list of businesses that fit this criteria are: eBay, Hotmail, PayPal, Half.com, and ICQ. I was wondering how many more are out there and which ones they are.

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