What are some alternatives?
When comparing Zabbix and Datadog, you can also consider the following products
LogicMonitor - LogicMonitor is the SaaS performance monitoring platform for the world's best IT teams. Deploy Fast, Monitor More, Improve Ops.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
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.
Sumo Logic - Sumo Logic is a secure, purpose-built cloud-based machine data analytics service that leverages big data for real-time IT insights
Icinga - Icinga is a fork of Nagios and is backward compatible.
PagerDuty - Cloud based monitoring service