I develop PredictSalary (https://predictsalary.com), a browser extension that can predict the salary range of job opportunities. It's free but one person told me that he didn't mind to pay me if I added support for job opportunities in his country (UK). The extension has 700+ users now. I don't know whether this number can be considered as significant user base or not.

Later, I'll add the capability to predict the salary range of people from their Linkedin profiles. I would make this a premium feature. Let's see how it goes.