Little Snitch
raboof/nethogs: Linux 'net top' tool
CommView
Colasoft - Network Analyzer, Packet Sniffer, Network Sniffer
Tshark
TCPDUMP & LIBPCAP
Ettercap
Endace Full Packet Capture Recording | 10-100Gbps & Beyond
jpr5/ngrep: ngrep is like GNU grep applied to the network layer. It's a PCAP-based tool that allows you to specify an extended regular or hexadecimal expression to match against data payloads of packets. It understands many kinds of protocols, including IPv4/6, TCP, UDP, ICMPv4/6, IGMP and Raw, across a wide variety of interface types, and understands BPF filter logic in the same fashion as more common packet sniffing tools, such as tcpdump and snoop.
Proxyman · Debug, intercept & mock HTTP with Proxyman
Fiddler - Web Debugging Proxy and Troubleshooting Tools
Npcap - Windows Packet Capture Library & Driver
Source-code: https://github.com/nmap/npcap
evilsocket/opensnitch · GitHub
Kismet - Wi-Fi, Bluetooth, RF, and more
Source-code: https://github.com/kismetwireless/kismet