Some love needed here:
Brew or MacPorts
Get your FOSS *nix utilities, apps and tools here. Both have binaries available but the latter prefers to build from source and is integrated in a more Unix/POSIX way. I love both, but I tend to end up back at Brew (despite its flaws) more often than not.
GPG Suite
Paid. Integrates into Mail and has a separate keychain for all your GPG/PGP keys. Allows transparent signing and encryption of all email, fully customisable, full key management (local and remote). Paid, but worth it if you don't want to wrestle with Thunderbird - the new integrated key management of which is a bit iffy.
Keka
FOSS. Archive manager for macOS. Does what it says on the tin. Download from the site for free, as the Store version is paid (as a form of donationware).
Maccy
FOSS. Excellent, light and very reliable clipboard manager for macOS. Perfect integration, immediate responsiveness from keyboard shortcut or menu bar, very customisable. Indispensable.
Murus
Paid. Excellent GUI for the native/built-in pf firewall. Makes setting up networks, rules and so on a breeze. Option for a menulet to show blocked connections, pf logs etc. Practically invisible, resource wise, as it's using the built in firewall and once the GUI is closed you're using no extra CPU or memory. It basically configures the native /etc/pf.conf config file and then exits. Superb bit of kit, especially on a laptop roaming across WiFi networks. The screenshots on the website are the old version - the newest version is much more modern, nicer and easier to use.
If you don't need a full on managed firewall or aren't confident in setting it up, then the sibling app
Scudo is more point and click - think Little Snitch but based on built in BSD/macOS tools. It's also a bit cheaper than Murus.
Sublime Text
Paid, but the 'trial' is unlimited (time and features) for unregistered devices. Pop up 'nag' screen every 100 saves (or something, I basically never see it). Fantastic text editor, full code style recognition (eg shell, js, yaml) and autocompletion. Very light and powerful - I've been using it for years and still don't use 10% of its feature set. I wish I could ditch TextEditor for this natively.
iStat Menus
Paid. System monitor for macOS. Fully and extensively customisable as a standalone app, menulet and notification centre widget. It lists all metrics (CPU, memory, hard disks, power/battery, networking, etc) with a list of graphs, hovering over each of which pops out lists of all associated stats. Very detailed, easy to read, lightweight. Been using it for years and find it indispensable.
Iterm2
FOSS. Fully featured and extensible terminal app that blows the native Terminal out of the water. Graphically accelerated, themable, does everything you'd want and more (including tmux, split screen, programmable outputs and more).
Rclone
FOSS. Remote file management and sync using an improved rsync. Multi-threaded, command line, GUI or web interface. Capable of mounting remote filesystems (eg Amazon AWS, Google Drive, OneDrive, PCloud) locally, and further able to create and mount encrypted volumes (AES256) within them for transparent end-to-end encryption regardless of your storage provider's abilities. Makes cloud storage safe. Fast and light. Cross-platform.
Following on from Rclone, and needing to be installed alongside it for it to function:
macFUSE
FOSS. File system utilities for non-natively supported filesystems like EXT* and NTFS. Accompanied by sshfs to mount locally a remote filesystem over SSH.