This is GERNOMIO, a datawarehouse to monitor Opennet wireless infrastructure.
You can query informations via this API using e.g. http://www.on-i.de/api/nodes/online
Currently this implements geoJSON answers only, that can be filtered by providing a spatial bounding box e.g. http://localhost:8080/api/links/online/?bbox=11.918641204833,54.039007590191,12.321358795167,54.120982290945