NetStumbler is a tool for Windows that allows you to detect Wireless Local Area Networks (WLANs) using 802.11b, 802.11a and 802.11g.
It has many uses:
Verify that your network is set up the way you intended.
Find locations with poor coverage in your WLAN.
Detect other networks that may be causing interference on
Detect unauthorized "rogue" access points in your workplace.
Help aim directional antennas for long-haul WLAN links.