Ronald Bradford wants to know what kind of Monitoring you use..
He specifically wants to know about Alerting tools
There's different cases , looking at it from a full infrastructure point my current favourite is Zabbix or good old Nagios,
But when looking at it from a debugging perspective you have MySQLAR or Hyperic, but those aren't in the alerting list.
However, when you are building HA clusters, you have custom scripts running either from mon or from pacemaker ..
Still .. Ronald probably wants more input :)