mCaptcha is a privacy-respecting captcha system based on proof-of-work. Instead of image puzzles, it has the visitor's browser perform a SHA256 computation, which makes attacks more costly for the attacker than for you. The system rate-limits requests without relying on IP addresses, so users behind a shared network address are not penalised, and it does not track or profile users. It offers APIs compatible with reCAPTCHA and hCaptcha so that it can replace them, and an administration panel for setting difficulty levels. You run the backend, written in Rust, yourself, for example with Docker.
mCaptcha is open source: the backend is licensed under AGPL-3.0, and client libraries use licences that also permit use in proprietary software. The project is developed by an independent community with no controlling company, with support from the NGI0 Entrust Fund via NLnet. Because you host the software yourself, you decide where the data resides and no foreign parent company has access. That suits European organisations that want full control. The project's demo instances are intended for trying it out only.