Wireshark is a network protocol analyzer that has become a standard across several industries. Supported protocols are still growing, the number going by the hundreds. It can do live captures and off-line analysis, VoIP analysis, and protocol decryption.
Coming with a three paned packed browser, this protocol analyzer reads and writes in many different capture file formats, exports output to XML, PostScript, CSV or plain text, and browses captured network data by the use of a GUI or of TShark utility.
Said to offer the most powerful display filters in the industry, this one can also decompress gzip files on the fly and display results in an intuitive manner, by the use of colors and packed listing.