In fact, there have been recent Zero-Day threats that Apple confirmed are in the wild that the Mac firewall and other built-in protocols could not defend against. In my experience there seem to be many Mac users who assume that the built-in security protections are sufficient protection against all threats. In many, if not most cases, it may prevent apps from performing essential functions, such as auto-updating software when that feature is enabled. And that can be a lot of apps if you like to use third-party apps, like many of us do. Regarding the recommendation by “abc” to configure the Mac firewall to “block all incoming connections,” if you do that it will cause problems with every app that relies on incoming network connections.
Similar to Lulu, it’s completely free and open-source so that’s awesome. It’s still in beta, but the app surprisingly works great. Netiquette is a latest network monitoring tool created by Objective-See team who have developed another app in this list, Lulu. Hands Off: Install (Free, one-time purchase of $49.99)
And the best part is, just like Little Snitch, you will receive a notification prompt whenever an app or service tries to make a connection. Apart from that, you can go deep and monitor what processes are trying to establish a connection with online servers.