Having an Iphone I often use the superb Google Maps. And, I believe as of version 1.1.3 of the Iphone firmware, you can press the ‘My location’ button and you find where you are. No GPS. Only cell tower data. And.. it is scaringly accurate.

Now you (at least I) figure.. how do they do it? And..surprise surprise.. you can find out here:
http://maps.alphadex.de/index.php?section=mylocation

I came by it here:
http://mrblog.org/2008/03/05/google-cell-tower-location-api-reverse-engineered/

As it says it that blog article apparently Google doesn’t want the world to now.. but it works stunning.