Vistumbler - Vistumbler is a wireless network scanner for Windows Vista and Windows 7. Features:
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.
Wifi Analyzer - Wifi Analyzer is a handy tool which helps you to select a better channel for your wireless router.
Waircut - Free network audit app
NetStumbler - Official source of information about Netstumbler.