Very simple web monitoring system that checks if a site is online and sends alerts by SMS If down. They also offer a free monit aggregation service. Plans start £3/month basic, £5/month for 5 sites every 30 seconds plus 15 sms credits / month.
A free hosted version of the open source monit-aggregator written by Matt Fawcett. Appears to be free with no node restriction.
Clever approach to monitoring mysql replication with monit. Ruby script checks replication is running and then touches a file, monit in turn monitors that file's timestamp and if it gets out of date, sends an alert. Sort of like a dead man switch. Clever.
Simple script to monitor mysql replication delay. It updates a value on the master every second and checks how far behind the value is on the slave. Simple yet effective.
A howto on setting up monit on an Ubuntu server. Fairly simple and straightforward. Includes basic info on customising alerts and so on.
Source code of monit-aggregator by Matt Fawcett / runslikeclockwork.com. He offers a free hosted version at that site.
A very useful SSL check utility which will support non standard port numbers. Useful for monit debugging on port 2812.