I've used a lot of different monitors (most listed here already) and Zabbix is by far the best. Lots of features. Graphing, Alerts, Script execution, Dependencies, Capable of monitoring anything and everything. -Steve
If you're going to set it up, learn how to use templates. Makes adding a lot of hosts with the same type of monitors and triggers real easy. Documentation is a little weak but the user forums are active.
Nice write up on Zabbix at Linux.com