How about PoS then? * Lock up, say, $5, in anonymous cryptocurrency like Monero * When visiting example.org, sign a message "example.org <current timestamp>"

How about PoS then? * Lock up, say, $5, in anonymous cryptocurrency like Monero * When visiting example.org, sign a message "example.org <current timestamp>" with your private key, and include the signed message in a request header * example.org can verify the signature (and the fact it's backed by $5) * example.org can also forward the signed messages to the advertiser as a proof of legitimate pageviews * if a bot farmer wants fake pageviews, they need to lock up separate $5 per bot (and if we're counting unique pageviews, then per pageview) edit: one gothca here is that sites could collect the signed messages to track users across sites. Perhaps there's a way to sign messages so that you prove you have $5, but not which $5 – I don't know.