[gvx]

I had the same idea. If you look at the sitekeys you can click "View deployment".

It's just a "I'm not a robot" checkbox. If you click it, it does some POW stuff in the background and that's it.

It's more of a rate limiter than a CAPTCHA provider like others have said, but given how cheap CAPTCHA-solving services are currently, I don't know if there's much of a difference in practice anyway.