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
netcat - Netcat is a featured networking utility which reads and writes data across network connections...
HTTP Headers - HTTP Headers allows you to quickly see the HTTP header information for the current URL.