With your help, OpenSignalMaps is creating 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 an Android application and uploaded to the project's servers, taking care to use as little processing power and battery life as possible.
You can use the project website to browse the data they've 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. And don't worry -- the data is stripped of any identifying information and available on a graphical interface to enable you to make sense of the raw data.