Monitoring network, LAN and bandwith activity for server

Is there any good software free or commercial that anyone can recommend to monitor our server.
We host over 100 websites on a dedicated server and all the 100 IP's are on the single NIC.  What I want to do is monitor traffic to each IP and total network traffic to the server.  We had a few issues with downtime intermittently and I'm trying to isolate the problem.  Any suggestions or recommendation would be really helpful.
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Monitor bandwidth, some provide more details than others.

Here is a summary of the recommendations:
1.  PRTG (two recommendations) -
2.  Websense -
3.  8e6  -
4.  mrtg -
5.  ntop-xtra -
6.  iPrism -
7.  NetFlow Analyzer -
If you are hosting that many sites, you may want to consider a good hardware firewall solution.
Check out the watchguard family of products.
Their logfile, bandwidth monitors, and reporting is wonderful!
Hope that helps.
CobraitiAuthor Commented:
The webserver sits behind our ISP's hardware firewall (Cisco) has been reliable. Except we don't get any reports. I was thinking more of software that runs on the server itself that monitors processes, LAN, system health, network traffic etc.
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