A field server is a sensor node which can create a wireless sensor network and simultaneously serve as Wi-Fi hotspots in open fields. Since 2001, we have been working on several Research and Development projects to develop field server technology and its applications. So far, three generations of field servers have been developed and are in use in more than a dozen countries for collaborative research and educational applications such as a global sensor network for Earth observation, food safety/IT-agriculture, advanced sensor technology in fields and image monitoring for urban areas. The field servers have proved to be quite robust for field applications. More work is in progress to develop applications for the field server and resolve issues with power consumption and increasing the communication range for certain applications