HTTP Debugger is a professional HTTP sniffer and analyzer for developers.
You can use HTTP Debugger to debug HTTP API calls to a back-end and between back-ends.
HTTP Debugger is very easy of use, with clean UI, and short ramp-up time.
It's not a proxy, and does not produce network issues!
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.