What are some alternatives?
When comparing Librato and Nagios, you can also consider the following products
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.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.
Pulseway - Pulseway changes the enterprise monitoring and management game with its mobile management solution.
Icinga - Icinga is a fork of Nagios and is backward compatible.
PagerDuty - Cloud based monitoring service