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API List - A collective list of APIs. Build something.
Cocoa Rest Client - A free open source, native Apple OS X app for testing HTTP/REST endpoints
Abstract APIs - Simple, powerful APIs for everyday dev tasks
gURL - gURL is a free, Open Source (GPL) HTTP request generator tool.
The Unsplash API - No-limits, do-what-you-want API for access to 200K+ HD pics.
Graphical HTTP Client - Graphical Http Client is a utility to help you test and interact with REST-based web services.