OpenCellID is the biggest open source database of Cell IDs (GPS positions of GSM cell towers) and is generated by a community similar to the OSM community.
The community mainly uses smartphone apps for contributing cell-id information to OpenCellID.
More than 26.400.000 cells have already been discovered worldwide, with more than 1.600.000.000 measurements (04/2016)
Data is available both from the API or downloadable completely as CSV files.
A German, English, French, Spanish and Polish version is available.