Power over ethernet.
Only four of the wires in the ethernet cable are used. The other four?
There's two ways they can be used. You can use a "cable doubler" which lets you run two lots of data over the same cable. I can see that would be great when you've strung insufficient cables; it's cheaper than doing another lot of stringing. But I've never had a use for it; I always string more cables that I think I'm going to need. So, for example, when I linked the garage in Chartridge Grange with the utility room where I had the most important servers and where my outside lines came in, I strung four overhead cables instead of the one that I actually needed. Because cable is cheaper than doing another lot of restringing. I never used the other three.
Recently, I read about POE, power over ethernet. I've known about this for a long time, but a light bulb went on in my head. In the Data Shed, there's a Raspberry Pi that gives me a GUI that lets me log in from there to anywhere (and it also does the in-house DNS, because all my internal computers have an IP that starts with 10.). That's powered by a power lead from one of my UPSes to a transformer that takes in 240 volts and gives out 17 volts, that goes through a step-down card to give 5.5 volts, and that goes to the Pi.
It occurred to me that I could get rid of most of that. So I bought a pair of POE splitter adaptors, £1.24 on Ebay. I took 12 volts from the PC power supply that powers all the other Pies, put that through an ethernet cable, split it out at the Pi end, put it through the step-down card to get 5.5 volts, and the Pi powered up just fine. Then I thought, hmm, how about using that same ethernet cable to connect it to the network? But the little piece of ethernet cable on the adaptor wasn't long enough to reach to the ethernet switch. Hmmm.
Then I remembered, a long long time ago, I bought ten female-female ethernet extenders. I used one of those, and two meters of ethernet, to connect to the switch.
And it all works perfectly!