Slashdot is discussing a story named BBC Crowdsources 3G Coverage Map.
Their summary: "The BBC is asking Android users to install an app which will upload information about 3G and 2G coverage, in order to build up a map showing where Britain has signal. The company behind the app, Epitiro, previously worked with the regulator Ofcom to measure 3G speed, and apparently found that O2 is slightly faster"
In the comments you'll learn that they have something similar for the Netherlands [in Dutch]. There's OpenSignalMaps too to display heat maps of signal strengths, but I'm not so sure about how "open" they are, since they provide the maps, but seems to keep for themselves the crowdsourced database (I'm wrong?). We mentioned in the past OpenCellID.org, which is truly open but doesn't display nice maps out of the box. On the same topic, we mentioned OpenBmap.org last April. Any other valuable source?