MacStumbler is a utility to display information about nearby 802.11b and 802.11g wireless access points. It is mainly designed to be a tool to help find access points while traveling, or to diagnose wireless network problems. Additionally, MacStumbler can be used for "wardriving", which involves co-ordinating with a GPS unit while traveling around to help produce a map of all access points in a given area.

Straight from the (http://www.macstumbler.com/ Official Website).