Missing FieldTest? It was a "secret" iPhone application that used to report all kinds of basic signal and connectivity details for your phone. You could access it by typing a special sequence (*3001#12345#*) into the Phone app. Field Test would list your local cell towers, their ids, relative strengths, and so forth.
Apple ditched Field Test in iOS 4, which made a lot of people sad. Having that information in-hand can really help with certain diagnostic uses of the phone, not to mention assist with home brew location calls. (Core Location has not been available from the command line, making home brew workarounds a handy asset.)
Now comes Signal. Created by iPhone devteam member planetbeing, Signal is now available on the Cydia store for $5. Signal brings back many Field Test style reports in a nicely designed package and adds some hot mapping action on top.
As you can see in the screen shot here and in the gallery below, Signal is like Field Test on steroids. It provides all kinds of cell information about the towers your phone interacts with.