Just started using Coverage Mapper
. From the author's website:
Coverage Mapper is a crowd sourced cellular coverage mapper. The Coverage Mapper application runs on your mobile device (currently only available for Android), collects signal strength data, tied to your location, and submits it to our server. With the data from many people, accurate coverage maps are generated, to give you a better idea of the coverage in areas you care about..
I've quite enjoyed driving around with the program running then uploading the data when I get back to my WiFi network (though it can also upload over 3G) and then looking back at the maps to see where coverage is good/bad.