Flapjack made a splash in the monitoring world circa 2010 as an alternative to Nagios, but without consistent development it soon went dormant.
In 2012 the Flapjack project was rebooted and changed focus from check execution (which other modern monitoring tools like Sensu do excellently), to event processing, notifications, and detecting emergent behaviours.
Flapjack now sits on top of existing monitoring engines like Nagios and Sensu, and provides an API for configuration, scheduling maintenance, acknowledging failures, and querying entity history (conveniently wrapped in a client library).
Flapjack can notify via PagerDuty, XMPP, and email, and also supports receiving acknowledgements from PagerDuty + XMPP.
In this talk Lindsay Holmwood takes you on a whirlwind demonstration of Flapjack's features, and explains where the project is heading.