BAsically, all you NEED is a laptop, a supported wlan card, and Netstumbler. Many people also add an antenna (w/pigtail) either omnidirectional, or directional, and a GPS so that netstumbler can "remember" the location of the access points it finds (and then upload or share these files). try searching the forums for "stumbler special" for a deal on the antenna\pigtail, and also read:
http://forums.netstumbler.com/showth...&threadid=1797
(The FAQ)