Capsa Enterprise Network Analyzer
I have not had to use network / protocol analyzers on any regular or extensive basis. On the occasions that I have had a reason to want to sniff the network and explore what was going on I have generally used Ethereal, which is now called Wireshark. With such a powerful tool available for free I have never had any reason to resort to commercial applications.
I have to say though that Capsa Enterprise could change my mind on that. I had an opportunity to work with Capsa Enterprise and was impressed with the product from installation to execution. While it isn’t free, it is reasonably priced and seems to be a solid value for the investment. Check out my review of Capsa Enterprise to learn more and for a link to the more in-depth white paper analysis.