Andiparos!

Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, and can be extended using any .NET language.

Charles is an HTTP proxy / HTTP monitor / Reverse Proxy that enables a developer to view all of the HTTP and SSL / HTTPS traffic between their machine and the Internet. This includes requests, responses and the HTTP headers (which contain the cookies and caching information).

The Zed Attack Proxy (ZAP) is an easy to use integrated penetration testing tool for finding vulnerabilities in web applications.It is designed to be used by people with a wide range of security experience and as such is ideal for developers and functional testers who are new to penetration testing.ZAP provides automated scanners as well as a set of tools that allow you to find security vulnerabilities manually.

apptalk.ninja is a great, easy-to-use tool for app developers tackle that monitors and debugs issues with their apps’ communication over-the-air.

Slowy is an OSX app which simulates custom connection's conditions and limits the network traffic. It is created for web developers that work on a local server, to test a website with a real-world connection.

Working quietly on the Menubar, Surge is a high-performance HTTP/SOCKS5 proxy server. It is capable of intercepting and logging the summary of HTTP traffic, and meanwhile transmitting the traffic to other proxy servers according to the flexible rule system. Multiple proxy protocols are supported including HTTP, HTTPS, SOCKS5, SOCK5 over TLS.

RESTed allows developers to quickly format and make HTTP requests and view the response.

Uthrottle is a tool which simulates custom connection’s conditions and limits the network traffic to a specified speed on network interfaces. In complex service oriented application stacks, some bugs only manifest themselves on congested or slow networking interfaces. so to test and make a slow environment to test them and know how they will act you need to slow down network interface speed and many other factors like latency rate etc With Uthrottle you can do all this very quickly and set a environment to test your projects . Application is able to throttle multiple interfaces at once and perform various actions regardless of any conflictions . It is like “Slowy” app for mac with more features and realtime interface updates and more. »

A Simple Windows application to help developers debug REST based services.