Ensuring high-quality service from one end user to another, over multiple interconnected networks, is central to the aims and approach of G?ANT2, the pan- European research and education network. In order to provide users with reliable and consistent performance monitoring data from multiple networks, G?ANT2's Performance Measurement and Monitoring joint research activity, in co-operation with Internet2 and ESnet, is developing a comprehensive network monitoring facility - perfSONAR. Within the framework of this activity, we have developed an open source, publicly accessible, easy to use, and yet powerful standalone graphical user interface client, capable of querying all available perfSONAR services and displaying the returned data in a concise manner. This paper gives a short overview of the existing stateof- the-art solutions, introduces the fundamental concepts of the perfSONAR infrastructure, and describes the design, implementation, and use cases of PerfsonarUI, while highlighting its main advantages. Possible future enhancements of PerfsonarUI are also outlined.