Ask HN: Any good FOSS alternative to Google's reCAPTCHA?

Google's reCAPTCHA is everywhere, they seem to have the monopoly of checking if the user's not a robot.

CAPTCHA systems are essentials to the web, and it seems important to me to have a (good) FOSS alternative, but I can't find any.

Are all CAPTCHA closed-source to make it harder for attackers? Am I missing something?