8 years later...
I have been looking for alternatives to Google's reCaptcha, which doesn't ruin the UX, tracks user, etc. and found this gem: Coinhive Captcha.
It works by mining Monero coins (hash count is adjustable) in the background and provides a server-side API to verify it. It should be noted, that - depending on the selected hash count to solve - it may be slow, specially on mobile devices.
