Here I will review some of the network monitoring tools I've been using for my work environment.

PHP Open Monitor
Requirements:
PHP and Apache.
Install:
Installation couldn't get any easier.
Overall:
Very lightweight for monitoring services on your Linux machines. This utility basically pings services of host machines. If the service is down the table will note it as down by the color red. If the service is up it's bright green!

-- site link
-- snap shot

Nagios
Nagios is an open source host, service and network monitoring program. Who uses it? Lots of people, including many big companies and organizations. Where can I get it? Right here. Can I get support for it? Yes! If that didn't satisfy you, click here to get answers to some of your basic questions about Nagios.

-- site link

Demarc
Requirements:
Snort, mySQL, and Perl
Install:
Installing Demarc takes about 10 minutes. Once install you can maintain and update everything through the Web based interface.
Overall:
I've used Demarc for a several weeks now and find it to be one of the all around best monitoring utilities for small to medium size lans. It's excellent at gathering and sorting all of your network traffic into readable graphs and tables. It also contains host monitoring utilities to let you know if a server goes down and will even email you. Last Demarc used MD5 to monitor file changes to any part of you ext2 file system.

-- site link
-- snap shots

ACiD
Requirements:
Snort, mySQL, and PHP.
Install:
The installation of this is actually very easy!
Overall:
ACiD is probably my second favorite monitor tool next to Demarc. This tool allows you to sort and analyze all of your network traffic and uses Snort and Mysql to collect and store all the network traffic.

-- site link
-- snap shot