Wikipedia

Software links:
PC/router:
NetStumbler
Kismet
EarthStumbler (converts NetStumbler files to KML for Google Earth)

Mobile:
WiFiFoFum (Windows Mobile) (Files can be natively saved as NS1, XML, KML, and more)
MiniStumbler (Windows CE)


Websites:
Wigle.net
WiFiMaps.com
Wardriving.com
Wireless related File Extensions and related tools

Check out a Google Map of a few spots I've captured on my HTC Touch Pro