Ask HN: Best non-reCAPTCHA CAPTCHA solution?

Does anyone have recommendations for privacy-respecting, hopefully open source, captcha solutions? I just want something low tech, usable and reasonably effective, to use on a signup form. Mostly interested in stopping dragnet spam.

I like the “balance the goat” thing that GitHub has, from arkoselabs, but it seems like you have to buy their whole enterprise anti-Fraud solution.

Does anyone know of a simple, effective option?