The best free alternative to Pingdom is Better Uptime. If that doesn't suit you, our users have ranked more than 100 alternatives to Pingdom and loads of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to Pingdom are Freshping (Free), UptimeRobot (Freemium), StatusCake (Freemium) and PHP Server Monitor (Free, Open Source).
AgentSlug.com is simple but perspective application to monitor your website. For now, you can find there a full featured uptime monitoring tool. However, the app is developed constantly to provide complementary webmaster tools in the nearest future.
It is an All-In-One Site Monitoring & SEO Tool that Reports site Performance, Monitoring Key aspects from Server Health to Domain Expiry, Title text and Website Uptime with instant Alerts to any Issues.
Netumo is a 24x7 Website Uptime monitor with Domain Tracking, so it can let you know when your site is down and back up and let you know when your domain is about to expire. You can get notified through email, SMS, Slack, Microsoft Teams, or Twitter.
Monitor your website, server, API, application, service and get notified if your resources have some problems with availability. Upzilla notifies you in Slack, Telegram, Messenger, or by email. Works also as a public status page with subscriptions.