This paper presents the Ubiquitous Sensor Network (USN) testbed, which is deployed at Electronics and Telecommunications Research Institute (ETRI), in Republic of Korea. USN testbed is a state-of-the-art sensor network experimental facility composed of various components. It includes heterogeneous sensor nodes with variety of sensing module ranging from ultra-low power to high performance hardware platforms, a scalable and reliable Ethernet based wired backplane network to program, interact with and receive/send data from/to the sensor nodes, an easily extendable application software suite which glues and drive everything in a user friendly manner, and an indoor and outdoor deployment space offering various environmental characteristics. We also included two case studies where researchers have put USN testbed into practice for experimental evaluation of algorithms and networking protocols. The case studies cover routing and clustering protocol in wireless sensor networks.