Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
ServerSuit - ServerSuit is a browser based program that enables remote Linux administration, monitoring, website hosting, and server setup automation.
DownDetector - A simple service to tell you whether a website is down.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Pingbreak - Pingbreak is an original and free monitoring service.
RedGate SQL Monitor - SQL Monitor helps you and your team find issues – before they become problems