According to Wikipedia:
Implicit crowdsourcing is less obvious because users do not necessarily know they are contributing, yet can still be very effective in completing certain tasks. Rather than users actively participating in solving a problem or providing information, implicit crowdsourcing involves users doing another task entirely where a third party gains information for another topic based on the user’s actions.
A good example of implicit crowdsourcing is the ESP game, where users guess what images are and then these labels are used to tag Google images. Another popular use of implicit crowdsourcing is through reCAPTCHA, which asks people to solve Captchas in order to prove they are human, and then provides Captchas from old books that cannot be deciphered by computers in order to try and digitize them for the web. Like Mechanical Turk, this task is simple for humans but would be incredibly difficult for computers.
Piggyback crowdsourcing can be seen most frequently by websites such as Google that data-mine a user’s search history and websites in order to discover keywords for ads, spelling corrections, and finding synonyms. In this way, users are unintentionally helping to modify existing systems, such as Google’s ad words.