problem and presents it to evaluate a user. For example, CAPTCHA tells human users from computer programs by presenting a problem that is supposedly easy for a human and difficult for a computer. While CAPTCHAs are effective security measures for preventing automated abuse of online services, the human effort spent solving them is otherwise wasted. The reCAPTCHA system makes use of these human cycles to help digitize books by presenting words from scanned old books that optical character recognition cannot decipher. (HC) Interactive online games: These are programs that extract knowledge from people in an entertaining way. (HC) "Human Swarming" or "Social Swarming". The UNU platform for human swarming establishes real-time closed-loop systems around groups of networked users molded after biological swarms, enabling human participants to behave as a unified collective intelligence. (NHC) Natural Human Computation involves leveraging existing human behavior to extract computationally significant work without disturbing that behavior. NHC is distinguished from other forms of human-based computation in that rather than involving outsourcing computational work to human activity by asking humans to perform novel computational tasks, it involves taking advantage of previously unnoticed computational significance in existing behavior. Incentives to participation In different human-based computation projects people are motivated by one or more of the following. Receiving a fair share of the result Direct monetary compensation (e.g. in Amazon Mechanical Turk, ChaCha Search guide, Mahalo.com Answers members) Opportunity to participate in the global information economy Desire to diversify their activity (e.g. "people aren't asked in their daily lives to be creative" ) Esthetic satisfaction Curiosity, desire to test if it works Volunteerism, desire to support a cause of the project Reciprocity, exchange, mutual help Desire to be entertained with the competitive or cooperative spirit of a game Desire to communicate and share knowledge Desire to share a user innovation to see if someone else can improve on it Desire to game the system and influence the final result Fun Increasing online reputation/recognition Many projects had explored various combinations of these incentives. See more information about motivation of participants in these projects in Kosorukoff, and Von Hippel. Human-based computation as a form of social organization Viewed as a form of social organization, human-based computation often surprisingly turns out to be more robust and productive than traditional organizations. The latter depend on obligations to maintain their more or less fixed structure, be functional and stable. Each of them is similar