Female-to-female RJ45 couplers ("join adapters" as described by the article) have been very handy in my experience. They're cheap enough that I don't mind leaving one now and again. It's so handy to be able to unplug something, splice in my laptop / tap / etc, and quickly return things they way I found them without rerouting the original cable. It's also handy when you don't have a long enough patch cable with you but you do have a couple shorter ones.
My current favorite is a 1 meter retractable USB C to C cable. Combined with the smaller GaN charger and the lower power requirement of newer MacBooks, I can pack really light, and that's great for bike and motorcycle travel.
I also have a set of USB adapters that plug into each other and let me charge anything with the same cable. Again, less stuff to pack.
A couple of my favorites:
15- and 25-foot power cables to eliminate outlet strips and extension cords in some installations.
Outlet strips with super long cords -- especially useful for avoiding the need to chain outlet strips in settings where doing so is considered to be a safety violation.
I love having so many little daptery-doos. My best hack is storing them though. I have a two tier desk, 24" deep main surface, and 10" monitor 'shelf'. I keep a half dozen tiny adhesive drawers under the shelf so they're always at hand.
One for flash storage + usb dongles, another with every usb adapter + short cables, audio things + usb power meters, pens, and other stuff. It is so nice being able to quickly use almost any device without a scavenger hunt first.
90 degree rotated RJ-45 male-male connectors would allow very easy RPi-like clusters.
Beware: most USB-to-SATA adapters do not work with HDDs, only SSDs
One USB A-to-C adapter I particularly like is a cable with adapters tied to each end, so you can use it for any combination of (A or C host) to (C or micro device).
FM transmitters you have to be slightly careful - they need to get power somehow; think about how that will happen.
Add to this list: small portable HDMI/VGA monitor for checking on headless machines.
If you use the various Ethernet adapters make sure you run some speed tests when you get it.. Those cheap dongles/converters ect are notorious for Working but cutting your speed and reliability of the connection.
Basicallly angled adaptors for everything.
Once upon a time in a galaxy far, far away my apple magnetic charger was right angled. Unfortunately the technology was forgotten and eventually lost with the passage of time.