SPAN (or RSPAN) you port of interest to one with a host running Wireshark (www.wireshark.org). Capture the traffic there and analyze it using that product. If you have an *nix host, grab the traffic using tcpdump and look at it raw or ftp the file to a host with wireshark - it can open tcpdump files.(Wireshark runs on *nix also.)

If you don't want to SPAN, you can use a splitter like a Gigamon to pull off optical traffic out of band.

Other options in include Sniffer / Remote Sniffer or Network Instruments' Observer.