Finally Open Signal Maps is ready. It's an app that shows you we're active cell towers and wifi routers are located. The idea is that knowing where these are will help you get better reception. It's alos pretty interesting to see where wifi routers are (even if the locations aren't 100% accurate).
The app is also part of the Open Signal Maps project... users can contribute data to help build TRUE maps of signal strength - don't just rely on what the networks say!
See http://www.opensignalmaps.com for more