I recently had a dilemma while working on a new product for Conspire Web Services.
The product in question Conspire Cordon is a tool that other developers can use to help secure their web scripts from external attack (a sort of "Script Firewall"). One of the motivations behind the project was to create something that can help make a small dent towards making the internet a better place.
With this community focused philosophy in mind, the puzzles started to popup. To help protect small business users that don't have the resources to throw endless money at a product, I wanted to offer the product for free (or at least some of the product).
The issue with a completely free product, is that the software requires server time to process any data thrown at it. On top of the hardware costs, there's also the time taken for a technician to periodically sit in front of a screen and flag any suspicious behaviour as new threats to the eco-system. There's a learning element to the tool, but there's also a level of manual oversight involved – it would be silly to just let it loose on the world without any control (it would probably decide that humanity itself is the threat and *somehow* spawn Skynet to cull the population).
The seemingly obvious solution was to follow the Google method and simply allow a small amount of usage per month and then have users pay for any overage after the fact. Conspire has the billing methods in place to handle such an arrangement (the voice service can be setup post-pay for instance).