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].

[0] https://groups.google.com/group/devops-toolchain/browse_thre... [1] http://www.youtube.com/watch?v=BXxtdE-Paco