Acrylic WiFi Scanner
Excellent WiFi scanner that helps you understand network interference problems, visualize WiFi channels in both 2.4GHz and 5GHz, assess the signals strengths of the different WiFi networks available, and show all neighbouring access points. Available in a free and commercial version.
PRTG Network Monitor
PRTG is a monitoring tool built by Paessler, a leading network monitoring software developer. It allows you to monitor all the systems, devices, traffic, and applications in your IT infrastructure, and come with highly interactive maps and dashboards.
NirSoft provides a useful collection of lightweight free utilities for Windows. These includes password recovery utilities, network monitoring tools, internet related utilities, Windows system tools, command line and desktop utilities.