KisMAC

This popular wireless stumbler for Mac OS X offers many of the features of its namesake Kismet, though the codebase is entirely different. Unlike console-based Kismet, KisMAC offers a pretty GUI and was around before Kismet was ported to OS X. It also offers mapping, Pcap-format import and logging, and even some decryption and deauthentication attacks.

inSSIDer

inSSIDer is a wireless network scanner for Windows, OS X, and Android. It was designed to overcome limitations of NetStumbler, namely not working well on 64-bit Windows and Windows Vista. inSSIDer can find open wireless access points, track signal strength over time, and save logs with GPS records.

inSSIDer

inSSIDer is a wireless network scanner for Windows, OS X, and Android. It was designed to overcome limitations of NetStumbler, namely not working well on 64-bit Windows and Windows Vista. inSSIDer can find open wireless access points, track signal strength over time, and save logs with GPS records.

NetStumbler

Netstumbler is the best known Windows tool for finding open wireless access points ("wardriving"). They also distribute a WinCE version for PDAs and such named MiniStumbler. The tool is currently free but Windows-only and no source code is provided. It uses a more active approach to finding WAPs than passive sniffers such as Kismet or KisMAC.

Kismet

Kismet is a console (ncurses) based 802.11 layer-2 wireless network detector, sniffer, and intrusion detection system. It identifies networks by passively sniffing (as opposed to more active tools such as NetStumbler), and can even decloak hidden (non-beaconing) networks if they are in use. It can automatically detect network IP blocks by sniffing TCP, UDP, ARP, and DHCP packets, log traffic in Wireshark/tcpdump compatible format, and even plot detected networks and estimated ranges on downloaded maps. As you might expect, this tool is commonly used for wardriving.