Fiddler

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.

Free Windows

Charles

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.

Commercial Mac OS X Windows Linux

mitmproxy

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.

Free Open Source Mac OS X Windows Linux

Zed Attack Proxy

The Zed Attack Proxy (ZAP) is an easy to use integrated penetration testing tool for finding vulnerabilities in web applications.

Free Open Source Mac OS X Windows Linux

HTTP Debugger

HTTP Debugger Pro - A Professional HTTP Sniffer and Analyzer for Developers

Commercial Windows

HTTPScoop

HTTP Scoop is a Mac network sniffer that displays requests to websites along with the response, and makes it fairly easy to see what's going on.

Commercial Mac OS X

Proxy.app

Proxy.app is the perfect tool for debugging and analysing both HTTP requests and responses and modify data in transit. It can be used to debug hard problems while doing...

Commercial