Background mining software via web browser

[buwaytress]

I noticed in this week alone two services now using "background mining" that runs via the web browsers of visitors to their websites. Piratebay was a little sneaky with it (announced in earlier in May on their blog but I didn't know until Spoetnik had a thread on it here). And today I noticed moonbitcoin faucet also offered the option to their users to increase payout bonus.

In either case, a max of 10 to 30% of the user's CPU would be used to mine (Piratebay for Monero and moon for Bitcoin).

Would anyone know where to get info on such a software? Do I need software (if open source) to run on a host server to manage this? Or is it just a simple script?