LuLu: free open-source macOS firewall
LuLu is the free open-source macOS firewall that aims to block unauthorized (outgoing) network traffic unless explicitly approved by the user:
Full details and usage instructions can be found here.
Feature

As in no ads, no time trials, no missing features. Because why not!?
And no, it doesn’t track, monitor, or spy on you – as that’d just be pure evil!
And no, it doesn’t track, monitor, or spy on you – as that’d just be pure evil!

The full source code for LuLu is available on GitHub. Such transparency allows anybody to audit its code, or understand exactly what is going on.

LuLu aims to alert you whenever an unauthorized network connection is attempted. As such, it can generically detect malware, or be used to block legitimate applications that may be transmitting private data to remote servers.

“Do one thing, do it well!” LuLu is designed as simply as possible. Sure this means complex features may not be available, but it also means it’s easier to use and has a smaller attack surface!

Want to know what network events are being detected? Or rules your users have added? LuLu provides simple mechanisms subscribe to such events, and stores data such as rules in an open, easily digestible manner.