OpenSignalMaps reports they have created:
…a comprehensive database of cell phone towers, cell phone signal strength readings, and Wi-Fi access points around the world. This data is collected via our Android application and uploaded to our servers, taking care to use as little processing power and battery life as possible.
You can use this website to browse the data collected, including heat maps that show exactly how strong signal is in any particular area, as well as all the nearby towers for your carrier.
