Wireshark

Wireshark is a popular open source protocol analyzer that allows users to capture and read info from applications such as Sniffer, Snoop and Microsoft network monitor. What makes this software stand out is the fact that you can download it and and install it free of charge. It can be successfully used for network error troubleshooting, communications and software protocol development, but also for network analysis. Initially the tool was released in May 2006 as Ethereal, but due to trademark issues it was renamed to Wireshark soon after.