Nagios (http://www.nagios.org) is probably the most popular open source problem detection and alerting system. It periodically checks services you define and compares the results to default or explicit thresholds. If the results are outside the limits, Nagios can execute a program and/or alert someone to the trouble. Nagios’s contact and alert system lets you escalate alerts to different contacts, change alerts or send them to different places depending on the time of day and other conditions, and honor scheduled downtime. Nagios also understands dependencies between services, so it won’t bother you about a MySQL instance being down when it notices t....