Coinhive

Cornhive is a cryptocurrency mining service that is designed to be installed on websites. It was originally designed for website owners to earn an income without using intrusive ads. Cornhive uses the website visitor's processing power to mine for cryptocurrencies such as Monero. The earned coins are then spit between the website's owner and Cornhive.

It is a dubious endeavor because the visitor doesn't know that the mining is taking place. Also most often the code is installed on hacked websites without the owner's knowledge.