You gave me an idea for a business model - Let's say you offer a pingdom-like service (to monitor your site availability and reliablity) -
You can just do the following:
1. Offer the service 100% Free, at your own risk
2. Offer a $x/month subscription . if you don't pay - you won't get any customer support.
Or, you can will still offer free support for non-paying customers, but without any priority (their tickets will be handled only after the paying customer's tickets)
Does anyone know of a company that is offering this type of model ?
(edit: added missing words and spacing)