inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
WiFi Scanner - A 802.11 wireless scanner and connection manager for Mac OS X.
Fing - Discover which devices are connected to any Wi-Fi network, map devices, detect intruders, assess...
WifiInfoView - Scans wireless networks in your area and displays extensive information, including: SSID, MAC Address, PHY Type (802.11g or 802.11n), RSSI, Signal Quality, Frequency, Channel Number, and more...
SoftPerfect Network Scanner - Powerful multipurpose network administration tool for Windows and macOS