List of Profitable Web Apps

[Cowboy_X]

A friend was asking about traits common to profitable web apps, and I realized that my own list was pretty short, and was unable to dig up any decent lists.

Our criteria is web apps that are profitable by virtue of users paying for access or accounts; apps that were free until MegaCorpDotCom bought them out and grossed the creators a hojillion dollars don’t count.

Any thoughts?