PRTG Network Monitor is an all-inclusive monitoring software solution developed by Paessler. Equipped with an easy-to-use, intuitive interface with a cutting-edge monitoring engine, PRTG Network Monitor optimizes connections and workloads as well as reduces operational costs by avoiding outages while saving time and controlling service level agreements (SLAs). The solution is packed with specialized monitoring features that include flexible alerting, cluster failover solution, distributed monitoring, in-depth reporting, maps and dashboards, and more.
Ethnose is a network sniffing application designed for teaching purposes. It will allow you a big insight of what is actually happening on your network and can be used for diagnostics (although it's rather meant to be an educational framework).