Bit-banging interfaces on a PC motherboard
It's usually easy to get computers to talk over ethernet, but making traditional PC hardware to talk to electronics or a GPIO line tends to be more difficult.
Issue came up when I wanted to make a smart wakeup switch for a local backup ARM-board the other day, which would toggle separate 5V power to it and its drive in-between weekly backups, so went on to explore the options there.
Simplest one is actually to wake up an ARM board using Wake-on-LAN, which can then control own and peripherals' power via its GPIO lines, but don't think mine has that.
Next easiest one (from my pov) seem to be grabbing any Arduino-ish MCU laying around, hook that up to a relay and make it into a "smart switch" via couple dozen lines of glue code.
Problem with that approach is that PC mobos tend not to have simple GPIO lines easily exposed and intended for use from userspace, but there are options:
Parallel port, if motherboard is ancient enough, or if hooked-up via one of cheap USB-to-LPT cables with tiny controller in the connector.
These have a bunch of essentially GPIO lines, albeit in a funny connector package, and with a weird voltage, but pretty sure only really old mobos have that built-in by now.
RS-232 COM port, likely via a header on the board.
Dunno about latest hardware, but at least ones from 6-7 years ago can still have these.
Even though it's a serial port, they have DTR and RTS pins, which are easy to control directly, using ioctl() with TIOCM_DTR and TIOCM_RTS flags on /dev/ttyS*.
Outside of pure bit-banging, RS-232 can usually be hooked-up to simpler hardware's UART pins via RS232-to-TTL chip, instead of just GPIO via optocoupler.
PC Speaker!
That's a 5V signal too (PWM, sure, but still no problem to read), and not like it's in much demand for anything else these days.
Might have to either emit/detect distinct frequency or sequence of beeps to make sure that's not the signal of machine booting on the other side.
3-pin fan headers with a voltage level controls.
Can be a neat use for those analog pins on arduinos, though not all monitoring chips are RE-ed enough to have linux drivers with control over fans, unfortunately.
Power on USB ports as a signal - at least some mobos can put those to sleep via sysfs, I think, usually with per-controller level of granularity.
For my purposes, COM was available via an easy pin header, and it was easy enough to use its control pins, with some simple voltage-level shifting (COM itself has -12V/+12V for 0/1).
Bit of a shame that PCs don't traditionally have robust GPIO headers for users, feel like that'd have enabled a lot of fancy cas modding, home automation, or got more people into understanding basic electronics, if nothing else.