Android: With mobile reception cycling through the news, one Android app deserves credit for giving a pretty darned accurate view of how connected your phone is. Antennas shows not only signal strength, but maps out your tower location(s).
You get the best reading of your signal strength if you're on a network that offers both 2G ("EDGE") and 3G connections, and you switch to 2G-only connection in your Android settings (under Wireless & Networks/Mobile networks). You'll see every tower that's connected to you, their individual strengths, and your overall connection strength. On 3G or CDMA networks, you'll only get to see one tower, due to an Android OS limitation, but you can still know where your strongest tower is located, and how you're doing on connection.
Antennas is a free download for Android systems only. A direct market link is shown here, and more details on the app at the link below. Thanks to OolonColluphid for the tip!