http://www.wanderfly.com/

http://namesake.com/

http://www.mashape.com/

http://mailchimp.com/

http://www.inkling.com/

http://www.airbnb.com/

I will add explanation later.

This blog post by Yee Lee, that explains basics of the viral distribution in a very approachable manner. 

Great stuff!

“Viral Loops typically center on:

1. Contact Lists — e.g., inviting new people thru the process of finding/adding new contacts to a contact list
2. Conversations — e.g., inviting new people by adding them to chats, events, groups, etc.
3. Media — e.g., inviting new people by sharing media/content with them
4. Incentives — e.g., inviting new people to unlock functional or financial incentives”

invite more people to get more out of the system. In this case get closer to the beginning of the line. Very smart. I am not sure if the line actually exists, but it would not kill me to get closer to the beginning.