Link: Signal – iPhone app
Signal is a utility created by iPhone Dev Team member planetbeing that displays information about the cell tower your iPhone is connected to as well as neighboring cells. It detects and maps the location of all nearby cell towers so that the user can easily get a good idea of their cellular coverage or where to go to get the best cellphone reception. Combined with the free iPhone carrier unlock that planetbeing has helped create, a user can swap SIMs and determine which carrier is best for him or her based on cellular coverage. In addition the map, other useful technical data is displayed including RSCP (signal strength) in dBm and Ec/N0 (signal quality) to all neighbors, the PSC of the cells, as well as the frequency each cell is broadcasting in.
Signal is exclusively available from the Cydia Store on jailbroken iPhone 3G, iPhone 3GS and iPhone 4.