Mockable - Quickly define some REST or SOAP endpoints and have them return json/xml (dummy) data.
Postman - The Collaboration Platform for API Development
WP Webhooks Pro - Do everything within WordPress without WordPress.
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.
Treespec - Test your webapp by recording a video of your workflow
WireMock Cloud - Fast and simple API mocking in the cloud, powered by WireMock.