ufw - Ufw stands for Uncomplicated Firewall, and is program for managing a netfilter firewall.
TinyWall - Lightweight and non-intrusive firewall
Gufw - Gufw is an easy, intuitive way to manage your Linux firewall.
GlassWire - Visualize network activity in detail, get notified when new apps access the network, look out for...
Little Snitch - Little Snitch is a firewall application that monitors and controls outbound internet traffic.
Firestarter - Firestarter is an Open Source visual firewall program.