Zabbix works great for this, is stable and whilst it's still a serious undertaking to properly configure it, it's at least not as bad as Nagios. It's also extr

Zabbix works great for this, is stable and whilst it's still a serious undertaking to properly configure it, it's at least not as bad as Nagios. It's also extremely flexible in how you deploy it (distributed monitoring, proxies, agents or direct checks) and what you monitor (custom scripts for oddball stuff are pretty easy to write for it, SNMP and many other common things are already included). If you want free/cheap hosted monitoring, have a look at Monitis (Warning: they were extremely clingy when it came to cancelling our account once we switched to Zabbix)