Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
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.
Uptrends - Uptrends is a website and mobile app that allows you to monitor your website. You can use it to both monitor the security and performance of your website as well as the activity on your site. Read more about Uptrends.
AppDynamics - Get real-time insight from your apps using Application Performance Management—how they’re being used, how they’re performing, where they need help.
Uptime.com - Everything you require for unified availability monitoring Simple & intuitive industry leading Enterprise-grade features delivered at a fair price, that are continuously improving.
Zabbix - Track, record, alert and visualize performance and availability of IT resources