InSSIDer

inSSIDer is a Wi-Fi network scanner application for Microsoft Windows and OS X developed by MetaGeek, LLC. It has received awards such as a 2008 Infoworld Bossie Award for "Best of Open Source Software in Networking", but as of inSSIDer 3, it is no longer open-source.

History
inSSIDer began as a replacement for NetStumbler, a popular Windows Wi-Fi scanner, which had not been actively developed for several years and reputedly did not work with modern 64-bit operating systems or versions of Windows higher than Windows XP. The project was inspired by Charles Putney on The Code Project.

Features

 * New in Version 5.0: channel utilization break down to show device (AP and client) airtime utilization; see connected client devices and info about client such as utilization and signal strength
 * Gathers information from wireless card and software
 * Helps choose the best wireless channel available
 * Wi-Fi network information such as SSID, MAC, vendor, data rate, signal strength, and security
 * Graphs signal strength over time
 * Shows which Wi-Fi network channels overlap

Windows

 * Version 5.0: Microsoft Windows 7 or higher
 * Version 3.0: Microsoft Windows XP SP3 or higher
 * Version 2.1: Microsoft Windows XP SP2
 * Microsoft .NET Framework 3.5 or higher

OS X

 * OS X Mountain Lion 10.8 or higher