PRTG - Paessler Router Traffic Grapher will get to you a graphical display of the trends and usage of bandwidth and network devices.
    As the name suggests this free to use Windows application will monitor and classify bandwidth usage. The well received data of live readings and network usage trends helps the administrator in optimizing the efficiency of network devices. Other than that the utility can also be used to monitor memory and CPU utilization. The software is exclusively designed to run on a Windows machine in your network; 24 hours a day, 7 days a week, and constantly monitor and record the network usage parameters. Features included are; support with the SNMP, packet sniffing and netflow protocol; works with all switches, routers, firewalls, as well as many other network devices; store the recorded data, and much more.
    This multi-functional tool works with SNMP, and tested with routers and switches from companies like Cisco, HP, 3Com, Linksys and DLink.