Antennas is a very simple but useful Android app, which is designed for those who wish know where the GSM towers to which their phones communicate with the network are located. The app also gives details about GSM/CDMA cellular status (and the RF strength). It works the best with 2G, as the 3G/CDMA support is limited by Android. Because of this, users are reporting a reduced accuracy on 3G/CDMA. However, on 2G it should be working just fine.
The app takes only a little over 35kb, and it works just perfect for me (I'm running on a 2.2 Nexus One on 2G). The towers seem to be located really accurate. It is possible to view their location in two ways, on a normal (Google) map or on a satellite map (Google Satellite).
You can download it from the Market by searching "Antennas" (first result, app created by mploy).