[yorick]

You may want to consider running some proof-of-work in the browser, that would be costly to run for bots.

[silasdavis]

That's not a bad idea.

I'd be concerned about being accused of/blocked for crypto-jacking. That said, I'd like to see a proof-of-work based captcha that donates any earnings to charity as cryptocurrency.

I came across hashcash.io before, previous discussion: https://news.ycombinator.com/item?id=7944540

Alternatively I could write something dead simple from scratch that looks for sha collisions to burn a few cycles. Would this be less dubious?

[ForHackernews]

All that does is further penalize the victims of botnets by driving up their electricity bills. It won't hurt the spammers who run the botnets.

[taffer]

This is only true if we assume that spammers are not already saturating the resources of their victims. And even then, it's better than penalizing blind or deaf people.