Loggr - Loggr offers cloud-based, real-time Web Application Monitoring, which includes monitoring application events and users with ad-hoc analytics and notification system.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Circonus - Circonus combines multiple monitoring, alerting, event processing, and analytics tools into one unified solution.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Sawmill - Sawmill is a universal log analysis/reporting tool for almost any log including web, media, email, security, network and application logs.
Prometheus - An open-source systems monitoring and alerting toolkit.