Spiceworks
Spiceworks is a lot of things -- market research, consulting, a tech-oriented community -- and a group of free tools that can provide insight into what's living on the network. The network visibility tools are part of a suite of functionality that include trouble ticket tracking and SSL certificate checking, but also include applications for scanning network devices, checking connectivity, and monitoring activity across the net.
Building a complete picture of the network begins with Inventory, the application that scans given network address ranges for any devices that exist. After that, the Connectivity Dashboard will help with ongoing monitoring of device and service connections, while the Port Scanner and Tester can provide a list of all the open and vulnerable ports on the network's devices.
Spiceworks is not a "freemium" model -- a tongue-in-cheek price guide shows a variety of free plans while promising to put you in touch with a sales consultant -- if you're lonely and just want to talk. As with so many of these products, Spiceworks cannot replace expertise, though the Spiceworks events and community can help your personnel get that expertise.
(Image: Spiceworks)