Checkmk - Checkmk - the software for effective IT monitoring
Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
Cloudlytics - Analyze Your Amazon S3 & CloudFront Logs.
Prometheus - An open-source systems monitoring and alerting toolkit.
Trafikito - Monitor CPU, RAM... or output of any command and make API call on event, default is email messages.