I needed a power supply that would power many different things for my bench test setup. And I did have a spare PC power supply so I thought it would be nice to use it and practical to have a 12V source but also have a regulated 5V and 3.3V source. The supply I have is a 650W unit and can supply up to 38A at 12V, 32A at 5V and 24A at 3.3V.
This is what I came up with (click on the image for a larger version):


The 2 connectors at the top right are to connect to the power supply: the one on the right is the 24-pin ATX connector and the one on the left is the 12V 8-pin connector. I have connected together all the 12V pins from those and divided the 12V supply into 5 independent circuits. The 5V supply has been divided into 2 circuits and there is one 3.3V circuit.
Each circuit has its own power switch and there is a main switch that enables the standard ATX pin to turn on the power supply. Each circuit has its own fuse (standard ATO fuse to have the largest amperage selection), 3 screw terminals and 5-pin header. There is a corresponding 3 screw terminal per circuit and a 36-pin header for ground; ground is common for all circuits.
The switches can handle up to 5A each so I have added a relay (ISO 280 footprint) for the first three 12V circuits. These can handle up to 35A each. The switch is connected to 12V and contact 86 on the relay and contact 85 is connected to ground. Contact 30 is connected to 12V and contact 87 is connected to the fuse. Also, I have put pin headers that connect between the switch and contact 86 and between contact 85 and ground. With shunts installed (as per the first picture), the relay is controlled by the switch directly. But it is possible to control either the + or - side from an external source by connecting a wire jumper instead of the shunt; for example I can connect a relay to the fuel pump output on a Megasquirt to power some injectors.
Finally, I have added large capacitors (1000uF) to filter noise; there are 2 on the 12V supply, and one each on the 5V and 3.3V supplies.
There are 4 mounting holes and I have installed 4 standoffs. Also, the standoffs hold a piece of cardboard so that the bottom is isolated; with all the exposed solder joints, it would be easy to have a short and some nice firework.
That is going to make powering my different test setups much simpler. And it also has the advantage of using a single supply for everything so there won't be issues with ground offset which always happens when using multiple supplies (as I needed to do before). And with screw terminals and pin headers, I can quickly and easily power up different types of circuits, boards and proto-boards.
Jean