Netstumbler (www.netstumbler.com - Free)

If you're a packet-level person, there are also versions of WireShark/Ethereal that use WinPcap that can catch wireless traffic.

Note: you still need the keys to decrypt the traffic. In the case of WPA, you have to be capturing when the WPA client attempts to associate, or you won't get the initial key (needed to catch the subsequent key).

Good Luck

Scott