Little Snitch - Little Snitch is a firewall application that monitors and controls outbound internet traffic.
TinyWall - Lightweight and non-intrusive firewall
Radio Silence - Radio Silence is a network monitor and firewall for the macOS. The software stops apps from making network connections. The firewall runs in the background and does not require an open window.
GlassWire - Visualize network activity in detail, get notified when new apps access the network, look out for...
Hands Off! - Monitor your computer. Protect your privacy.
Windows Firewall Control - Windows Firewall Control is not the built in firewall system in the Windows operating systems.