http://en.wikipedia.org/wiki/Software_as_a_service

http://en.wikipedia.org/wiki/Freemium

http://blogs.amrresearch.com/enterprisesoftware/2009/01/inside-a-startup-who-needs-an-it-department-not-zuora.html

http://peripateticaxiom.blogspot.com/2010/01/its-screaming.html