Network Traffic Analyser

Would anyone be able to suggest a good, pref free network analyser or packet sniffer for a network. We are about to implement a F5 Network Load balancer and I would like to take an in depth look at network traffic. I have toyed with a couple but they seem to be limited to traffic that is only coming or going to the single PC’s nic. I would like to see all traffic on our subnet.
thanks