RapidSpike!

Monitors your websites every 5 minutes, totally free. Get alerts by e-mail, SMS, Twitter or RSS. Add up to 50 websites. Monitors can be refreshed for an instant check with a click. Supports https and authentication with user/password.

Pingdom web site monitoring and other infrastructure for you and instantly let you know when anything goes wrong. Measure uptime, downtime, and the performance of your website as experienced by visitors.

Rollbar collects errors that happen in your application, notifies you, and analyzes them so you can debug and fix them. JavaScript, Ruby, Python, PHP, Node.js, Android, Flash, .NET, and Java libraries are available.

Datadog is a monitoring service for IT, Dev & Ops teams who write and run applications at scale, and want to turn the massive amounts of data produced by their apps, tools and services into a unified view of IT infrastructure. Datadog's capabilities are provided on a SaaS-based data analytics platform that enables multiple teams to work collaboratively on infrastructure issues.

Logmatic.io is an Operations Data Platform for Log and Machine events. We help companies improve their software and business performance by leveraging their machine data.

PHP Server Monitor is a script that checks whether your websites and servers are up and running. It comes with a web based user interface where you can manage your services and websites, and you can manage users for each server with a mobile number and email address.

Grafana provides a powerful and elegant way to create, explore, and share dashboards and data with your team and the world.

Cabot is a free, open-source, self-hosted infrastructure monitoring platform that provides some of the best features of PagerDuty, Server Density, Pingdom and Nagios without their cost and complexity.

Monitors the performance of your websites, DNS, and email servers along with blacklist monitoring. Get alerts by e-mail and SMS. Add 1 monitor for free. Plus free online network and DNS lookup tools.

Mist.io is an open source software and a freemium service that helps you manage and monitor virtual machines (VMs) across multiple public, private and hybrid clouds using your mobile phone, tablet or laptop.