In no particular order.
I've tried many different note-taking tools over the years, but since late 2018 I'm a Notion convert. Things I like the most: cross-platform support (incl. web and mobile) with cloud synching, block-based editing with support for a wide-variety of block-types, frequent releases with new features.
iTerm2 is a widely popular replacement for macOS Terminal with a large feature-set. Chances are you're already using it if you're reading this :-)
Super-fast replacement for grep with sane defaults. Must-have.
Bat - A modern cat clone with line syntax highlighting, line numbering and more. Highly recommend to set alias cat='bat'.
Visual Studio Code has become my go-to general purpose text editor. I use it for general text editing as well as coding. It performs very well on small to mid-sized code-bases and I find it (much) better than Sublime, BBEdit, TextWrangler or Atom. I exclusively use the beta-quality Insiders Build which is generally very stable.
Cross-platform mind-mapping tool. While I don't often use mind-mapping tools, I've tried out quite a few mindmapping tools over the years, and settled on Simplemind because it's cloud-synching and cross platform support, its rich feature-set and the fact that it's actively maintained.
Automation framework that allows users to write Lua scripts to interact with macOS APIs. You can find a few of my hammerspoon scripts on github.
General purpose command-line fuzzy finder. I used it primarily as much better replacement for bash's default history search feature (i.e. CTRL+R).
Much improved diff tool. Especially useful in combination with git.
From the jq website: "jq is like sed for JSON data - you can use it to slice and filter and map and transform structured data with the same ease that sed, awk, grep and friends let you play with text."
I use jq all the time when working with REST data. Especially powerful in combination with curl.
Clipboard manager for mac. Does the job, nothing fancy. I'm currently in the early stages of writing my own clipboard manager for macOS geared towards developers, since most other clipboard managers I've tried haven't fully met what I'm looking for.
Cross-platform password manager with large feature-set. I was a fairly early adopter, and after using it for about 10 years on a daily basis, I'm still a very happy user.
Database IDE that can connect to many different database backends.
Utility app that allows you to snap windows into organized tiles - similar to how window snapping works on Windows. Useful to put screens next to each-other.
Menu tray icon organizer - very useful to de-clutter your tray menu.
Better menu bar clock with calendar and time zones.
Drop-in replacement for SSH that deals with intermittent server connectivity. Gone are the annoying typing delays when SSHing to a server over WAN or a spotty internet connection.
Tiny program that helps you to prevent your Mac from automatically going to sleep or starting a screensaver.
Screenshot capture and annotation tool - use it every day. Currently evaluating Annotate as a replacement since Skitch's future is unclear.
Automatically closes inactive tabs and makes it easy to get them back. Life-saver to keep your browser window somewhat clean (and computer fast!).
Powerful vector-based diagramming tool.