Rarely-used macOS software
DaisyDisk provides a graphical overview of how much storage is in use, making a 256GB SSD feasible.
Carbon Copy Cloner takes all of the data on my laptop and mirrors it to an attached storage drive, but I don't use it very often.
Kaleidoscope, a text difference viewer, is receiving updates again, but usually the terminal diff view is sufficient.
Hex Fiend helps me dig around in binary files to see how they're structured.
I wish I had more reason to use the excellent Transmit, but I rarely use FTP to transfer files between machines or servers.
Xcode for Objective-C and Swift programming, where its auto-completion and test integration cannot be beat.
BBEdit for its search and replace interfaces, excellent text difference viewer, and comforting Mac-ness.
nvi for commit messages and quickly browsing C code.
Graphics
Despite being presentation software, Keynote has worked well for sketching out diagrams before I commit them to something like Monodraw or Mermaid. In a pinch, I also use it as a text outliner.
I've settled on The SketchUp 3D modeler for woodworking projects and and Fusion 360 for 3D prints, but I'm not thrilled with either of them. SketchUp is a delight to work in, but I'm still using the 2017 desktop app because I don't use it enough to commit to a Pro subscription. The outliner is a key feature that's missing from its current free offering. Fusion 360 is just too much power that I won't use and seems very complex. I wanted to use SolveSpace, but it's missing some features and it's extremely unintuitive, with few learning resources available.
I just discovered a new, inexpensive vector art program called Amadine that seems like a promising alternative to Sketch or the moonlighting pen tools of other graphics packages.
For a better experience with file formats I use often, Gapplin renders SVGs in a native Mac UI, fstl is a very fast STL viewer, and Keka acts as a front-end for decompressing different archive types.