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.