If you want to map out your area's hotspots, war-driving is the best way to do it. One of the popular applications used is Kismet – "an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system."
Here's a tutorial on how to use Kismet on Ubuntu as a war-driving tool with GPS.
A word of warning, though. War-driving does not mean that you can piggyback on the open access point for your internet needs. Depending on where you are, it may be illegal to do so. Regardless of where you are, I believe it is outright unethical to use open access points without asking for permission. :)
[photo courtesy of nim.]