Sounds like Sensu (https://github.com/sensu/sensu) fits the requirements. It's a small monitoring framework that works in conjunction with Chef or Puppet, PagerDuty, Librato, Graphite, and others.
Sensu came up several times in the recent monitoring discussion on devops-toolchain [0]. I'm going to try a setup with sensu, graphite, and collectd soon- like the one Sean Escriva described at ChefConf [1].
