openBmap is a free and open map of wireless communicating objects (e.g. cellular antenna, Wi-Fi, Bluetooth). It provides tools to mutualize data, create and access this map.
It is currently available via static downloads, under creative commons Attribution-Share Alike 3.0 Unported, most of the data are harvested through special apps installed on handheld mobile units (cell phones, tablets, etc), though some data have been imported (and maintained on regular basis)
The site also offer coverage maps, with OpenStreetMap as base (google map is also available as a choice)