Kismet is a free and open Wi-Fi stumbler, packet sniffer, and intrusion detection system for Windows, Mac OS X, Linux, and BSD. It shows the AP details, including the SSID of “hidden” networks. Plus it reports the noise levels and gives you the signal-to-noise (SNR) values. It can also capture the raw wireless packets to a PCAP file, so you can import into Wireshark, TCPdump, and other tools.
Kismet, however, in Windows only works with CACE AirPcap wireless adapters due to the limitation of Windows drivers. It does, however, support a variety of wireless adapters in Mac OS X and Linux.