Cloudlytics - Analyze Your Amazon S3 & CloudFront Logs.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Qloudstat - Analytics & monitoring for access logs in Amazon S3, Amazon CloudFront, Google Cloud Storage and Rackspace CloudFiles (Akamai CDN)
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.
Checkmk - Checkmk - the software for effective IT monitoring
Prometheus - An open-source systems monitoring and alerting toolkit.