Healthchecks.io - Monitor your cron jobs and scheduled tasks, get notified when they fail.
EasyCron - Get frustrated with Cron on your server? Hosting limits your Cron use?
PushMon - PushMon is a new kind of cloud-based monitoring system.
crontab guru - The quick and simple editor for cron schedule expressions by Cronitor
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
corntab - CRONTAB EDITING MADE SIMPLE