A micropayment is a financial transaction involving a very small sum of money and usually one that occurs online. PayPal defines a micropayment as a transaction of less than 12 USD while Visa prefers transactions under 20 Australian dollars, and while micropayments were originally envisioned to involve much smaller sums of money, practical systems to allow transactions of less than 1 USD have seen little success.
One problem that has prevented the emergence of micropayment systems is a need to keep costs for individual transactions low, which is impractical when transacting such small sums even if the transaction fee is just a few cents.
Micropayments were initially devised as a way of allowing the sale of online content and were envisioned to involve small sums of only a few cents. These transactions would enable people to sell content on the Internet and would be an alternative to advertising revenue.
During the late 1990s, there was a movement to create microtransaction standards, and the World Wide Web Consortium (W3C) worked on incorporating micropayments into HTML even going as far as to suggest the embedding of payment-request information in HTTP error codes. The W3C has since stopped its efforts in this area, and micropayments have not become a widely used method of selling content over the Internet.
Early research and systems 
In the late 1990s, established companies like IBM and Compaq had microtransaction divisions, and research on micropayments and micropayment standards was performed at Carnegie Mellon and by the World Wide Web Consortium.
IBM Micro Payments 
IBM's Micro Payments was established c. 1999, and were it to have become operational would have "allowed vendors and merchants to sell content, information, and services over the Internet for amounts as low as one cent".
An early attempt at making micropayments work, iPIN was a 1998 venture-capital-funded startup that provided services that allowed purchasers to add incremental micropayment charges to their existing bill for Internet services. Debuting in 1999, its service was never widely adopted.
Millicent, originally a project of Digital Equipment Corporation, was a micropayment system that was to support transactions from as small as 1/10 of a cent up to $5.00. It grew out of The Millicent Protocol for Inexpensive Electronic Commerce, which was presented at the 1995 World Wide Web Conference in Boston, but the project became associated with Compaq after that company purchased Digital Equipment Corporation. The payment system utilized symmetric cryptography.
The NetBill electronic commerce project at Carnegie Mellon university researched distributed transaction processing systems and developed protocols and software to support payment for goods and services over the Internet. It featured pre-paid accounts from which micropayment charges could be drawn. Initiated in 1997, NetBill seems to have died completely sometime after 2005.
Online gaming 
Recent micropayment systems 
Current systems either allow many micropayments but charge your phone bill one lump sum or use funded wallets.
Flattr is a micropayment system (more specifically, a microdonation system) which launched in August, 2010. Actual bank transactions and overhead costs are involved only on funds withdrawn from the recipient's accounts.
A short-lived micropayment system set up by Visa Inc in Australia, Payclick relied on a funded wallet that was drawn from when purchases at participating online retailers were made. This service was discontinued on 12 February 2013.
Znak it! 
An online content monetization platform that uses pre-paid virtual tokens that allow access to content, such as articles, videos, games, etc. It may be popular in Poland.
PayPal MicroPayments is a micropayment system that charges payments to user's PayPal account and allows transactions of less than US$12 to take place. The service is, as of 2013, offered in select currencies only.
- Micropayments paypal.com
- Visa launches new way to pay online payclick.com.au, 24 June 2010 (archive.org cache)
- In Online World, Pocket Change Is Not Easily Spent nytimes.com, August 27, 2007
- Micropayments Overview w3c.com
- Toward a Click-and-Pay Standard wired.com, 11.03.99
- Common Markup for micropayment per-fee-links 1.1 Origin and Goals W3C Working Draft 25 August 1999
- Archives of IBM Micro Payment sites archive.org
- IBM Micro Payments (Archive) archive.org
- Johnson, Amy Hellen. "iPIN". ComputerWorld.com. ComputerWorld. Retrieved 4 January 2012.
- Compaq to license digital cash technology cnet.com, December 23, 1998 6:10 PM PS
- Millicent (Archive) archive.org
- Millicent What's New – June 1997 (Archive) archive.org
- 2.6.10 Micro Payments (micropay) bof Current Meeting Report, November 8th 1999 Internet Engineering Task Force – ietf.org
- The NetBill Project (Archive) archive.org
- About NetBill (Archive) archive.org
- Archives of Netbill sites archive.org
- Steve O'Hear (August 12, 2010). "Flattr opens to the public, now anybody can ‘Like’ a site with real money". TechCrunch Europe. Retrieved August 13, 2010.
- "MCoin Product Lines - Mobile Marketing magazine". Mobile Marketing Magazine. 2011-06-30. Retrieved 2011-07-02.
- "Cancellation of service". payclick by Visa. Visa. 2012. Retrieved January 05, 2013.
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- Fear not! Let’s get you on the right path, my friend: Consumers Zong Official Site
- For purchase of virtual goods, see "Zong Lets You Bill Web Apps To Your Phone". TechCrunch. 8 September 2008.
- For use in games and social networks, see Where to find Zong Zong Official Site
- Rao, Leena (October 26th, 2010). "PayPal Unveils Micropayments For Digital Goods, Facebook Signs Up". techcrunch.com. AOL. Retrieved November 23, 2012.
- "Micropayments". PayPal Integration Center. PayPal. Retrieved November 23, 2012.