Have a look at Ethereal ( "http://www.ethereal.com":http://www.ethereal.com ) which can be downloaded for free. Ethereal is a free network protocol analyzer for Unix and Windows. It allows you to examine data from a live network or from a capture file on disk. You can interactively browse the capture data, viewing summary and detail information for each packet. Ethereal has several powerful features, including a rich display filter language and the ability to view the reconstructed stream of a TCP session.
You are right. Solarwinds is a full SNMP-based Management Tool! Nevertheless SNMP is a good way to monitor network load. For example Switches with RMON (Remote Monitoring) deliver a lot of informations about the actual network traffic. An easy and inexpensive way to access these data is the use of a SNMP-OPC Server. Than you are able to "access" these data in HMI/SCADA-Systems (i.e WinCC, Genesis32, iFix etc.)or the Microsoft Office Suite. Have a look at "www.snmp-opc-gateway.com":http://www.snmp-opc-gateway.com .