You may own one or more old computer towers with an ATX power supply that just gather dust, but since the ATX connector provides +12V, -12V, 5V, 3.3V and GND signal it could be converted into an bench power supply. People have been doing this for a while, but elliotboney’s solution looks neater than most with a 3D printed part that’s attached on top of the power supply.
I basically kept 3 wires of each voltage, a 3 black wires for each ground. So 3 yellow for 12V, 3 Red for 5V and 3 Orange for 3V. Any extra wires I cut as close to the board as possible and put a little piece of heat shrink over the cut wires just to prevent shorts.
For the switch, you’ll use the green wire and a ground. Internally, you’ll want to connect the gray wire and an extra 5V (red) wire. This is the power_ok to let the PSU know it’s outputting the correct voltage.
That post clearly explains ATX to bench power supply conversion in details, and provides a ATX connector pinout diagram.
Once all the connections are done, the back of the 3D printed part looks like that:
Via Adafruit Blog.