tools for monitoring network traffic

Do you know of any good tools for monitoring network traffic (preferably free) ? I'm hoping to get a better idea of any bottle-necks etc. with a view to possibly upgrading to Gigabit switches.

Network is windows server2003 (3) and 80 client pc's (xp). 100mb switches. All branded hardware HP and Dell.
kr1zAsked:
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michkoCommented:
Take a look at ntop.  
http://www.ntop.org/overview.html
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Network Analysis

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