Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.
CloudShark - CloudShark is a browser-based threat assessment tool.
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
PCAPdroid - PCAPdroid is an android app to capture the phone traffic and analyze it remotely (e.g. via Wireshark). The traffic can be easily downloaded from a remote device thanks to the integrated HTTP server, or streamed to a remote UDP receiver.