Open Source Software is software that can be obtained as both executable binary files and the source code used to produce it. Proprietary, closed source, software is distributed only as the executable application binaries. Open Source Software provides not only an application, but to those who can read the code, the knowledge of how the application functions.
In scientific applications, the provision of knowledge is a must; you need to know what your software is doing to your data, for example, for your results to be repeatable, testable and valid. As software applications, suites and environments used by archaeologists become ever more complex, the need to be able to demonstrate exactly what an application is doing with your data is a necessity; the potential for this is provided only by Open Source Software.
Open Source Software presents the opportunity to modify existing applications to make them fit the needs of archaeologists better; as such we can provide complex and feature-rich applications to what is a relatively small discipline without requiring the resources of an international software development company. By the same token, Open Source Software affords us both choice and flexibility; you're never "locked into" Open Source applications in the way that proprietary software can sometimes do.
Finally, Open Source Software is often provided at no cost to the end user. As such everyone has access to the same tools and the results we produce are limited not by budgets, but by our own skills and imaginations; Open Source Software presents a level playing field. More importantly, Open Source Software removes redundant spending from software procurement models; if you invest money into new features in an Open Source application, they can be enjoyed for all users. Similarly, enhancements funded by others can be used by yourself; nobody ever pays for the next must-have feature twice. For archaeology, where budgets are often stretched, this is element of Open Source Software is critical. In short, it allows archaeologists to work together to develop software for our needs at a fraction of the cost of paying someone else for ill fitting applications that were designed for others.
Mozilla's Firefox is one of the best known Open Source applications and is measurably better than its closed source competitors. Similarly, the OpenOffice.org suite provides a fantastically developed range of office applications that better proprietary counterparts and costs nothing to run. These are just two examples, however, in fact almost every closed source application available has a comparable Open Source alternative, often available for free download.
Open Source Software is being adopted in great numbers by businesses and governments around the World; its appeal to archaeologists should be particularly strong. As such, Open Source is fundamental to the Open Archaeology approach.