If I had to give some requirements....
- Free (as in speech and beer) or low cost
- SNMP capable
- ping / fping options
- GUI (http, I'd guess) of some sort for monitoring
- TCP connect options
Checkout http://www.opennms.org/ It may suit your needs,
Craig
Ditto on OpenNMS. Rock solid system and very fast. It can be kind of a bear to configure (XML configuration files and testing even simple changes sometimes require you to restart the entire monitoring system which can take a while), but once you get the thing set up right, it just works.
It does graphing as well, but I prefer to use Cacti for that.
Ray