Of course now that I want it, I can't find a reference for it . .
On of the issues in doing a project like this for free, is that many customers equate the cost of an item or service with its value. In the case Anonymous Coward mentioned above the system/website has no cost so some will feel that is has no value. It can also extend to the persons time to develop/maintain the system. As the system grows, change requests are submitted that 'cost' the developer but, add no cost or value in the eyes of the customer.
Several companies that offered support for 'free' services discovered that customers from the free service were much more demanding and intolerant that those that paid even a nominal fee for the same or a similar service. A few companies even discontinued their 'free' offerings as the cost to maintain those services became too burdensome.