Portmaster is a free and open-source privacy suite that puts you back in charge of your computer.

Visualize network activity in detail, get notified when new apps access the network, look out for malware, and block badly behaving apps.

A firewall protects your computer against unwanted guests from the Internet. But who protects your private data from being sent out? Little Snitch does!

LuLu is the free, shared-source macOS firewall that aims to block unknown outgoing connections unless explicitly approved by the user.

OpenSnitch is a GNU/Linux application firewall.

As soon as you’re connected to the Internet, your apps can send whatever they want to wherever they want, unnoticed by you, because all of this happens invisibly under the hood.

NetHogs is a small 'net top' tool. Instead of breaking the traffic down per protocol or per subnet, like most tools do, it groups bandwidth by process.