This paper presents an empirical analysis of the Open Source development process from the point of view of the involvement of the developers in the production process. The study focuses on how developers contribute to projects in terms of involvement, size and kind of their contribution. Data have been collected from 53 Open Source projects and target application domains include different areas: web and application servers, databases, operating systems, and window managers. Collected data include the number of developers, patterns of code modifications, and evolution over the time of size and complexity. The results of this study show evidence that there are recurrent patterns in Open Source software development and these patterns are common to all the projects considered even if there are no superimposed processes for development, application domains are different, and there are contributions from people spread across the world.