For web servers you might charge per request served. For text editors you might charge per unit time the editor is being used.
It'd be interesting if the maintainer could define an acceptable salary, and all "consumers" would just split the bill by proportion of their usage according to the kind of metrics mentioned above.
I would gladly pay such a bill if nearly all the money went straight to the developer, I could cap my contributions, and it was extremely easy to opt-in. Ideally the package manager would read some dotfile or ask me! The problem with projects like Bountysource is that people will never track down all the OSS projects they depend on and figure what would be a reasonable donation to give them. Too much agency is required of the user to achieve meaningful adoption.
Has anything like this been attempted?
Edit: clarifications ...