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I use one of the free analyzers mentioned in the article in the link posted by jimlynch. Capsa (the free version) has worked well for me. The free version only allows you to analyze one network adapter at a time, so it is fine for small businesses like mine, but larger companies might want to pay the $500 or so that the full version costs so you can look at multiple adapters at once. It is easy to use, assuming you have a working knowledge of internet protocols, and I actually found it fun to use for digging down and seeing usage patterns. You can configure the settings in a variety of ways and it can tip you off to patterns that suggest things like DoS attacks, your network being used for spamming, etc. Capsa was a good choice for me, especially considering that it was free.