Fing - Discover which devices are connected to any Wi-Fi network, map devices, detect intruders, assess...
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Advanced IP Scanner - Advanced IP Scanner shows all network devices, gives you access to shared folders, and can even remotely switch computers off. Download it Free.
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...
Zenmap - Zenmap is the official cross-platform GUI for the Nmap Security Scanner.