Many new methods and tools have been developed to measure the quality of network paths for the last ten years. However, there are relatively few works that consider deploying these methods for collaborative network measurement: a number of measuring points belonging to different autonomous systems collaborate on monitoring and diagnosing their network performance. In this paper, we present Planetopus, a distributed system for facilitating collaborative network monitoring. Planetopus provides a single platform for configuring and scheduling measurement tasks performed on a set of distributed measuring points. Planetopus currently performs measurement mainly using OneProbe and tcptraceroute. Moreover, we introduce two useful facilities for analyzing the measurement data: a new metric for quantifying route changes and a heatmap-based visualization method for discovering patterns and anomalies from a set of path measurements. We demonstrate the utility of Planetopus through several case studies in which poor routes are identified and corrected, different ISPs' network services are compared, and network performance problems are diagnosed.