This is a great tool, a powerfull Linux sniffer, that lets you even sniff layer two packets.
All you need to do for using it is (as root) enter this command
tcpdump -i any -l
If you want to listen on any interface of you PC (could not work in Promiscuous mode) and put the output on the screen.
tcpdump -i eth0 -w file
This will instruc tcpdume to work only with etho (-i eth0) and to write to file the output for later analisys.
After capturing the file you can use Ethereal to graphic the results.
