Social network analysis (SNA) has attracted a lot of attention over the past years. Existing tools for SNA do not allow a user-centric analysis of the social neighborhood, i.e., the subgraph of the user's friends and friends of a friend. In this paper, we introduce SONAR, an open source Web application for user-centric SNA. Its extensible architecture and flexible data model allows developers to embed SONAR directly into social networking websites. A performance evaluation shows that our application scales well with the number of users and adds only minimal overhead to the SNA algorithms.