NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
Wifi Analyzer - Wifi Analyzer is a handy tool which helps you to select a better channel for your wireless router.
Vistumbler - Vistumbler is a wireless network scanner for Windows Vista and Windows 7. Features:
WiFi Scanner - A 802.11 wireless scanner and connection manager for Mac OS X.
NetStumbler - Official source of information about Netstumbler.