Open Prediction Markets (this site) is focused on creating an easy to use tool for building Prediction Markets that is licensed under an Open Source license. There are other open source tools* which do this so you might call this "yet another open source prediction market" project.
Why Prediction Markets?
Prediction markets go by many names: event derivatives, prediction games, idea futures, virtual markets, information markets, decision markets. They are a form of derivatives - a security whose value is based on the value of something else - that are defined by the market. A popular form of event derivatives is for political elections such as the president of a country. Markets in general are a great means to aggregate the wisdom of large numbers of people. Stock market indexes are generally considered to be "leading indicators" of the economy in general. Prediction markets harness this "leading indicator" capability for any event.
Why Open Source?
Open Source software that is "free as in conversation and free as in beer" enables you to download the tool for yourself, set it up, and use it without any payment. You are also free to modify it and share it with anyone else. These liberties ensure the survival of the tool beyond any one company or developer. If this isn't important to you, just smile that you get it for free.
Basing the tool on Drupal means that many of the features are already built (users, permissions, commenting). It also means that the software will run on most servers which goes along with the goal of making prediction markets accessible to anyone. Drupal (or more precisely PHP+MySQL/PostgreSQL) is relatively easy to install and easy to learn. I hope that these characteristics will lead to wide adoption and make it easy for people to contribute back the improvements that they make.
Existing Open Source Prediction Markets
Perhaps the most famous open source prediction market is Zocalo an MIT Licensed project written primarily in Java. A slightly less attractive site is the IdeaFutures project which is written in Perl with a BDB database and licensed under the RECIPROCAL PUBLIC LICENSE - note that some purists do not believe the Reciprocal Public License to be truly "Open Source." Marmix which is MozillaPublicLicense1.0. Serotonin which is GPL.