amon - Self-hosted server and app monitoring tool
Cloudlytics - Analyze Your Amazon S3 & CloudFront Logs.
S3stat - S3stat is a service that takes the detailed server access logs provided by Amazon's CloudFront and Simple Storage Service (S3), and translates them into human readable statistics, reports and graphs.
Checkmk - Checkmk - the software for effective IT monitoring
Trafikito - Monitor CPU, RAM... or output of any command and make API call on event, default is email messages.
Zabbix - Track, record, alert and visualize performance and availability of IT resources