Finding cell phone signals with SignalMap
SignalMap hopes to alleviate any pain in this area. They have used the Google Maps API and mapped up a user generated list of cell phone signals by major service providers, mapping dead spots and comparing signal strengths.
There are a lot of mapped signal areas, but it is not by any means a comprehensive guide at the moment. However, it's building into a great site to check before heading out on business or pleasure to check if you might be heading into a bad area. To help the directory grow users can add their signal strengths, phone type and carrier to the list.