Vast numbers of free/open source software (F/OSS) development projects use hosting sites such as Java.net and SourceForge.net. These sites provide each project with a variety of software repositories (e.g. repositories for source code sharing, bug tracking, discussions, etc.) as a media for communication and collaboration. They tend to focus on supporting rich collaboration among members in each project. However, a majority of hosted projects are relatively small projects consisting of few developers and often need more resources for solving problems. In order to support cross-project knowledge collaboration in F/OSS development, we have been developing tools to collect data of projects and developers at SourceForge, and to visualize the relationship among them using the techniques of collaborative filtering and social networks. The tools help a developer identify “who should I ask?” and “what can I ask?” and so on. In this paper, we report a case study of applying the tools to F/OSS projects data collected from SourceForge and how effective the tools can be used for helping cross-project knowledge collaboration.