StatusOk (Open source) - Get notified through Slack/email when your website is down
SE Ranking - All-in-One cloud-based SEO software for agencies, marketing teams and webmasters
Better Uptime - We call you when your website goes down
DeskAlerts - Send alerts to any corporate device: PCs, phones, tablets, etc.
Is it down by Freshping - Check if any website is down from 5 global locations
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.