PRTG Network Monitor
Runs on a Windows machine within a network, collecting various statistics from the machines, software and devices that are designated. It can also auto-discover them, helping map out a network. It also retains the data to see historical performance, helping react to changes. Comes with an interface with point-and-click configuration. Easily share data from it with non-technical colleagues and customers, including live graphs and custom reports. Plan for network expansion and see what applications are using network resources.