Monitority - Simple website monitoring service (sms/email alerts)
Beeceptor - Unblock yourself from API dependencies, and build & integrate with APIs fast. Beeceptor helps you build a mock Rest API in a few seconds.
Suprvise - Know instantly when your sites are down.
Webhook.site - Instantly generate a free, unique URL and email address to test, inspect, and automate (with a visual workflow editor and scripts) incoming HTTP requests and emails.
PingPing - A simple tool to monitor your website and SSL
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.