Bosun: Stack Exchange's Open Source Monitoring and Alerting System

We open sourced this a couple months ago. It is similar in nature to Prometheus (So I thought I would post this now) but using OpenTSDB as the primary backend. It also support sGraphite and Logstash as backends. It is largely focused on alerting and includes the ability to test alerts against history.

It is also cross platform - runs on Windows and Linux. Optionally you can use our monitoring agent scollector with it that monitors Windows and Linux as well as common applications that run on them.