Proxy apps

MitmProxy

Free and open source interactive HTTPS proxy.
https://mitmproxy.org
https://github.com/mitmproxy/mitmproxy

Charles

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).
https://charlesproxy.com

HTTP Toolkit

Intercept, view & edit any HTTP traffic.
https://httptoolkit.com

Proxyman

Capture HTTP(s) in a few clicks
Best-in-class native macOS app to capture, decrypt, and mock your HTTP(s) requests/ responses with powerful debugging tools.
https://proxyman.com

Fiddler

Web and HTTP Debugging and Troubleshooting Made Simple
Uncover software bugs when HTTP communication is involved. The Fiddler set of tools helps you promptly identify errors to effectively fix them.
https://telerik.com/fiddler

WireShark

Wireshark lets you dive deep into your network traffic, free and open source.
https://wireshark.org