mCaptcha is an open-source CAPTCHA system based on proof of work. Instead of asking visitors to identify pictures, the browser performs a computation and receives a token that the website’s backend must verify.
You can host the service yourself and integrate its widget and verification API into your application. Test the computational difficulty on the devices your visitors use and review the current release status before deployment; the project documentation describes active development rather than a stable release.
Source: Product overview
