DevRaven enables engineering teams or individual developers to setup active monitoring for their services/applications and get alerted when things don't work as expected.
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Postman - The Collaboration Platform for API Development
Beeceptor - Unblock yourself from API dependencies, and build & integrate with APIs fast. Beeceptor helps you build a mock Rest API in a few seconds.
Checkly - Fresh API monitoring and Site Transaction Monitoring. From one great dashboard.
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.
Dynatrace Synthetic Monitoring - Dynatrace Synthetic Monitoring offers a synthetic test for insight into application.
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.