Mining cryptocurrencies is a great way to earn small amounts of residual income (that could possibly be worth much more in the future). This is becoming an alternative revenue source some websites (and people with more nefarious intentions) are banking on.

Salon.com, for example, is giving readers the option to opt out of advertisements in exchange for letting the site use their CPUs to mine Monero, a cryptocurrency that's easier to mine than Bitcoin and has a current market cap of $4.6 billion.

Of course, where there's room for a legitimate way to earn money, corruption is soon to follow.

https://www.cnet.com/how-to/how-to-stop-sites-from-using-your-cpu-to-mine-coins