Open source & free software
Open source software is software whose source code has been made available to the general public, often under a free license such as GNU or Apache. Free or open source software is an example of commons-based peer production. This software is produced via a shared effort on a non-proprietary model. This means that many people together on a project for a variety of reasons without the product being attributed to any one person or segment of the group. Each participant retains copyright on their contribution but grants license to anyone. Consequently, the software is fully within the public domain, and (depending on the license) may be freely modified or redistributed so that everyone may benefit from the entire project but no one can claim exclusive rights to all of it.
Open source software has greatly affected the reception of peer production as many businesses, like Google, use open source software not because it is free but rather because it is reliable and other businesses, like IBM, use free software because it improves the equipment and services they offer.
The Firefox browser is another example of open source free software. Users have the ability to download add ons or plug ins to modify their browser to their own personal tastes. Beyond just changing the browser's skin, users can download add-ons that block pop-ups, download videos, translate pages, and a whole host of other abilities.
Open source software is a different term than free software. Free software refers to the software you can get for no price at all and the issue with software you must pay for. Open source software, on the other hand, is software that is free but also contains some software you must buy and committ to (semi-free software).
Benkler, Yochai. The Wealth of Networks. New Haven: Yale University Press,2006
Free Software, Free Society: The Selected Essays of Richard M. Stallman: http://www.gnu.org/philosophy/free-software-for-freedom.html