LuLu is a small, shared-source macOS application level firewall that’s finally reached v1.0. Unlike other macOS firewall solutions, LuLu is 100% free, with no ads and no trial version.
I’ve been using it on my home Mac Mini for a while, and it works perfectly — Most of the time I don’t even notice it’s there, until a new application or service tries to access anything on the network when it pops up asking if that’s ok. Perfect.
For more details, check the official site.