I'm not sure about anyone else, but I've been using MRTG for years now. I've been monitoring bandwidth trends, CPU usage, Memory usage, etc.....

Now you can use it as your IDS

The Multi Router Traffic Grapher (MRTG) is a simple cross-platform tool that administrators have used for years to monitor network traffic loads. The concept is simple: it queries SNMP counters and creates HTML pages with live network graphs showing bytes coming in and bytes going out. MRTG can show much more than in and out traffic, it can graph any SNMP counter. Microsoft has a web site that demonstrates some of the many SNMP counters available on a Windows 2003 server.
This article was found on here.


I would still rather use snort or something similar.....