Wireshark is a free tool that is used for analyzing the traffic of computer networks such as Ethernet, FDDI, PPP, Token-Ring, etc.
Among advantages of the tool over the similar programs are graphical user interface and various filters.
Wireshark can capture the traffic in an online mode, it allows to save and look through the previous traffic.
The tool saves captured data in various capture file formats, it has many protocol decoders like TELNET, FTP, POP, RLOGIN, ICQ, SMB, MySQL, HTTP, NNTP, X11, NAPSTER, IRC, RIP, BGP, SOCKS 5, IMAP 4, VNC, LDAP, NFS, etc.