Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
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.