In general it is assumed that a software product evolves within the authoring company or group of developers that develop the project. However, in some cases different groups of developers make the software evolve in different directions, a situation which is commonly known as a fork. In the case of free software, although forking is a practice that is considered as a last resort, it is inherent to the four freedoms. This paper tries to shed some light on the practice of forking. Therefore, we have identified significant forks, several hundreds in total, and have studied them in depth. Among the issues that have been analyzed for each fork is the date when the forking occurred, the reason of the fork, and the outcome of the fork, i.e., if the original or the forking project are still developed. Our investigation shows, among other results, that forks occur in every software domain, that they have become more frequent in recent years, and that very few forks merge with the original project.