Zabbix

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.

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.