Network stumbler is a wifi scanner and monitor tool for windows, it allows you to detect WLANs using 802.11a, 802.11b and 802.11g wireless cards.
Its targeted to network professionals however is widely used by networking enthusiasts and hackers because it helps you find non broadcasting wireless networks.
Among it uses are:
1.- Verify that our network is well configured.
2.- Study the signal strength or coverage on different points of our network.
3.- Detect interference between one or more wireless networks.
4.- Its really useful to point directional antennas for long distance links or just put the wireless card and antenna in the best signal point.
5.- Can be used to detect non authorized connections.
6.- Wardriving, which means detect all routers near to your network.
It offers same of the functions mentioned above for GPS networks, obviously using the correct antennas, however isnt commonly used to achieve this.
Unfortunately it doesnt support all wireless cards and Windows Vista isnt officially supported, thought some people reports that it works fine, I didnt have the same luck and it didnt work on my laptop.
If you need Net Stumbler functionality in Vista, try Vista Stumbler!