Every Mac owner should visit the website of Mr. Patrick Wardle, Objective-See.
As he says on his website, his softwares are simple but effective. They aim to fill the gaps in the MacOS security without bloat. Below you will find the best product that Objective See share with everyone for free:
LuLu - Firewall
LuLu is a firewall for every outgoing connexions (which are not monitored by the Apple System). If you are conscious of privacy matters, it is the first step you should take. Indeed, every program on your Mac is sending request on the Internet. With this tool, you can control which connexions you allow and which not. Of course, the majority are necessary for your computer to work so you will lose some time accepting new connexions. But the more you work with LuLu, the simpler it is as you save your choices for later.
Do Not Disturb - For paranoid
A simple software to find out if your machine is touched by someone else while you are away. Every time your MacBook is slipped open, it will take a picture allowing you to check if your mates or any intruder try to use it.
KnockKnock - Explore every software installed on your Mac
If anything is installed on your Mac, KnockKnock will find it out and test it with VirusTotal. KnockKnock will show you lists of software by categories: Authorization Plugins, Browser Extensions, Cron Jobs, Directory Service Plugins, Event rules, Extensions and Widgets, Kernel extensions, Launch Items, Library Inserts, Library Proxies, Login Items, Login/Lockout hooks, Periodic scripts, Quicklook plugins, Spotlight importers and Startup Scripts.
Personally, we run it once a year to clear launch daemons or plugins that uninstalled software may have left behind (such as TeamViewer, Adobe, VPNs, ...).
TaskExplorer - Explore every process running on your Mac
TaskExplorer will check every process that are currently running on your Mac and test it with VirusTotal; like a super-charged Activity Monitor.
Reikey - To know if your keyboard is tapped
For people that fear that someone may try to forge their password by registering every key you type on your keyboard. This little software will alert you if any process running on your Mac tries to save your keyboard activity.
BlockBlock (beta) - Alert for malwares
BlockBlock is running in the background of your computer, looking for malware that try to install itself persistently (monitors files and process). It will alert you whenever some persistent component is added to your Mac.
Note: you should not check all the options in the settings tab, because "Passive Mode" will allow programs to install without noticing you.
RansomWhere? - Alert for file encryption
RansomWhere? is running in the background of your computer as well and will monitor your files in case some of them are being encrypted without your knowledge (as would do a Ransom malware). If any software that is not whitelisted encrypts data, RansomWhere? will alert you in real time. You even have the option to terminate the process.
OverSight - Control your microphone and camera
OverSight will monitor the use of your camera and microphone, allowing you to deny a process from using them. This can be a little painful if you are doing lot of calls or videoconferences.
Netiquette - Show every connexion between your Mac and Internet
Not really a must-have software but we like Netiquette that allows to trace every connexion your computer is doing on the Internet.
Objective See rocks
These softwares are real rock stars, available for free. If they are valuables for you, do not forget to tip the author.
Thus, be aware that professional attackers can by-pass any protection if they know they are installed on your computer.