[CentOS] network monitoring systems

I use Argus for monitoring and alerting: http://argus.tcp4me.com
Initially I was almost going to write a full essay about how
much and why I like it :) but a pic worths a thousands words.
Just check out the demo, and you'll get a pretty good ideea
about what it can do. The configuration files are also very
nice and clean, you can break the configuration into several 
files, group the options as you see fit, etc.

Here's a glimpse of how the config files look like,
it can be as simple as:

    Host "alex" {
        hostname: localhost
        Service TCP/SSH
        Service TCP/SMTP
        Service TCP/POP
        Service TCP/IMAP
        Service TCP/HTTP
        Service UDP/Domain/slack.i
        Service UDP/NFS
    }

or as complete as you need:

    Host "alex" {
        note: This is the main monitoring server, located in Oradea, Romania.
        details: The system is a PIII @600 MHz box with 256RAM, 2xSCSI disks.

        graph: yes
        hostname: localhost
        Service TCP/SSH
        Group "Email" {
            Service TCP/SMTP {
                label: SMTP
            }
            Service TCP/POP {
                label: POP3
            }
            Service TCP/IMAP {
                label: IMAP
            }
        }
        Service TCP/HTTP

        # UDP/DNS sends a 'status-query', UDP/DNSQ sends an 'IN ANY' query.
        # Some DNS servers (notably djbdns) do not handle 'status' queries.
        # UDP/DNSQ is available only in versions 3.1 and later.
        Service UDP/DNSQ {
            hostname: 192.168.0.1
        }
        Service DNSQ/A/slack.i {
            hostname: 192.168.0.1
            expect:     192.168.0.1
        }
        Service UDP/NFS
        Service Prog {
            severity: major
            label: Storage
            command: /usr/bin/nc localhost filestat
            expect: OK
        }
        Service Prog {
            label: MySQL
            command: /usr/bin/nc localhost mysql-ping
            expect: 2500
        }
        Service Ping {
            uname: alive
        }
    }


Good luck,
Alex