Although open source-projects existed since the early 1990s Wikipedia has had a jump start and a nearly exponential growth of members. Both belong to virtual groups, a generic term for every group of people who is communicating over the Internet. Virtual groups could be divided into emotional-oriented and into work-oriented virtual groups. Wikipedia operators and open source-projects are virtual groups of the work-oriented kind. In my speech I want to compare open source-projects especially KDE and the Wikipedia project. There are common and different characteristics, which I want to discuss by means of social psychology. Both were founded from software developers and belong to a social movement. Both share therefore a common belief: the free flow of knowledge and information and both have could be seen as social groups which communicate via Internet. It seems that there are more common practises than differences at IT-infrastructure, group boundary and recruitment, division in background and foreground, and inner (deciding) structure of the group. While comparing I will also take a look upon how open source-projects and wikipedia protect their public good. The public good is protected in open source-projects through mailing lists and versioning systems with distributed writing access, a reputation hierarchy, special roles/positions, etc. There are minor variances between open source-projects and Wikipedia: at Wikipedia for example a closed writing access is against their beliefs.