The trigger was a machine in my rack that wouldn't boot and wouldn't show me why. No display, no remote console, just a box sulking three feet from the floor while I crouched in front of it with a monitor balanced on one knee. A proper IPMI card costs more than the server it would live in, so I built a KVM-over-IP out of a Raspberry Pi instead.
The software is PiKVM, which turns a Pi into a device that captures the target's HDMI output, emulates a USB keyboard and mouse, and serves the whole thing as a web page you reach from anywhere. You get the BIOS, the boot menu, the kernel panic, all the things a normal remote login can never show you because the machine hasn't got that far yet.
The parts list is short. A Pi 4, an HDMI-to-CSI capture bridge so the video comes in through the camera connector with low latency, and the USB OTG port wired to the target so the Pi can pretend to be a keyboard. PiKVM ships a flashable image, so beyond wiring there's barely any setup: write the image, boot it, point a browser at it.
The fiddly bit is the USB. The Pi's data USB port doubles as the OTG gadget port, and getting the target to see it as a keyboard and mass-storage device meant getting the cabling right and trusting the docs when the first attempt did nothing. Once it caught, though, it caught completely. I can mount an ISO over the network and the target boots from it as if I'd plugged in a USB stick, which means I can reinstall an OS on a headless box without touching it.
The real test came a few days later when the sulking machine sulked again. This time I opened a browser, watched it POST, saw it choke on a failing drive in the boot order, dropped into the firmware and disabled the port, all from the sofa. No crouching, no monitor on the knee. For a Pi I had spare and a capture stick that cost less than a takeaway, it has comprehensively paid for itself. The only downside is that I've now removed the last reason I ever had to get up and look at the rack, which my step count has noticed.