There has been discussion before about proof of human work coins (
https://bitcointalksearch.org/topic/what-are-the-hurdles-to-creating-a-proof-of-human-work-coin-1966878). A coin based on proof of human work would not necessarily be a full fledged currency like bitcoin. Rather it could be used in similar ways to points, incentives, free trials and free usage tiers in a way this is anonymous and does not require each adopter to create their own implementation. This proof of work coin would basically be earned by solving captcha type puzzles and could only be spent once. I believe currently, Google's select the squares where the image contains an X, Y or Z is a great candidate for the type of puzzle. A couple use-cases would be:
An anonymous online service who wants to provide a free-tier access, but also wants to prevent abuse. The online service would simply charge users this human-proof-of work coin for the entry level tier. They would only need to implement the existing wallet for this coin instead of developing a whole new scheme for themselves.
An online crypto-casino could accept this human-proof-of-work coin as an alternative to a faucet.
However i do foresee potential abuse vectors, such as human captcha solving farms. But someone would either have to pay these farmers to generate the coin for them, or the farmers would trade the coins for other currencies . This would give the human-proof-of-work coin a monetary value. Small perhaps, but that value would simply be the basis of the value of whatever free access is given. So a person who wants to access a website who accepts the human-proof-of-work coin could in theory buy some from the "farmers market" at their market value, or simply produce it themselves for free.