Type in your street address, city, state and zip code at AntennaSearch.com. It has detailed information on over 1.9 million towers and antennas in the United States from the Federal Communications Commission.
This web service helps to pinpoint existing towers as well as small, hidden antennas in order to determine cell phone coverage in a given area. It also shows the locations of future towers which are still in the planning process. The service is updated on a weekly basis. Another service which is available is CellReception.com/towers. This site has combined the cell tower data with Google Maps to create a convenient graphic interface.