Packet sniffers do exactly what the name implies, they allow you to watch the packets that are touching your host. For good they extremely helpful in debugging problems with networks. They can also be used for several malicious purposes including spying on people and harvesting passwords.
- ethereal download A GUI packet sniffer that works on all major operating systems (UBO, MacOSX, Windows etc)
- tcpdump download A CLI sniffer that works on all major operating systems (UBO, MacOSX, Windows (with pcap)
- snoop A CLI sniffer for Solaris