Freeware Monitoring Packages

Possible agent candidates

mon- Service Monitoring Daemon
Looks like a good candidate for becoming a cluster-watching agent.
SWATCH: The Simple WATCHer
Expanded syslog()-ing to central "logmaster" server, monitoring of such logs for alarms. Can send mail, pipe messages to a program, or exec a program on alarms. Could be another agent

Packages to consider for Design Ideas

big brother
monitor local client/multiple display,pager server system. Many platforms supported. Checks accessibility with ping, availability of services (http, ftp, sntp, dns, telnet, ssh, etc.) Disk space, CPU usage, important processes. Support for custom external tests, many provided.
The Angel Network Monitor
Simple system polling monitor, writes pretty html page showing systems.o Plugin checks for tcp servers up, disk space, system load, ping times.

Also seen...

http://home.att.net/~rongage/dallas.html
[fm] ctm
[fm] ucd-snmp
[fm] snmpd (scotty)
[fm] tkload
Tkload - Tk load monitoring tool
[fm] autostatus
[fm] bindmon
[fm] mon