Ethereal is one of the best open source packet sniffer available today. It is available on both Windows and Unix Platform. 
Detailed list of Ethereal features can be found here !
Richard Sharpe and Ed Warnicke have written an extensive user guide(450 pages) on Ethereal. The intent of the document was to improve the accessibility of Ethereal.
If you are new to Ethereal or if you are already using Ethereal this guide is for you 
Download Ethereal User Guide V1.1
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