Proxy.app!

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.

mitmproxy is an SSL-capable man-in-the-middle proxy for HTTP. It provides a console interface that allows traffic flows to be inspected and edited on the fly. It also features mitmdump, a commandline tool that provides a tcpdump-like interface for saving, viewing and manipulating HTTP traffic.

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.

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.