There is captchatrader, which is sort of like that - you can "store" captchas by solving a lot of them at once. This earns you credit which you can use later to have your captchas solved for you.

See this: http://www.ghacks.net/2011/08/19/captcha-trader-automatic-fr...

Of course any such service can be used by spammers, so there is no real "white-hat"/"black-hat" distinction.