You've seen other ATX power supply mods here on instructables, but this is my version, a bit less refined but it looks nice and most importantly, it works.

Step 1: Cut Off Those Cursed Molex Connectors!

Tools: Screwdrivers (phillips)
Drill
Wire Strippers
Needle nose pliers
Linemans Pliers
Electical tape / heat shrink
One LED of the desired color (and a 1/4 ohm resistor if you feel the need)
Scrap wire
Spray paint
Sandpaper

This took me around an hour to complete, a bit longer to document and mess with minor hiccups i encountered.

Well, for the most part if your working with an ATX power supply, it has a 20 pin motherboard connector on it, cut all of your wires about 1 foot long, the brown, gray, and purple wires arent necessary,and can be cut shorter, but do NOT cut the green wire, that will go to our switch.

Step 2: Connect All of the Wires of the Same Color

Separate and bundle all of your wires by color, excluding the green and one black wire (they will go to your on off switch). Cut a hole ( i used a dremel) and mount your switch (im not sure what it should be rated for, the switch i ended up using is a push button style, i salvaged from my broken Sega Dreamcast power supply, i would assume it is rated for 120 VAC). I thenI taped all of my wires together for a clean look, although if i had binding posts i would much rather use them than the wires setup i chose.

Step 3: LED Mounting

I then took my 3mm 3VDC LED and attached leads from the black and orange wires and fed them back into the case of the power supply, and soldered on my LED. I then hot glued the LED to a scrap piece of aluminum. This allows the LED to serve as both a Power on/off light, and as i later found out the capacitors inside of the power supply retain a charge and after being disconnected from the power source and the light will stay lit. (helpful to know if you need to open up your power supply case, because you should ALWAYS discharge capacitors by shorting them before doing any electrical work near them.)

Step 4: Crimping Wires and Adding Aligator Clips

Well, the title sums this up, the idea is to bundle all of your wires (i grouped 5 wires per crimp) and using the crimps i had, i created the necessary electrical connections and plan to later add alligator clips on the ends for convenience. I didnt in the pictures shown, it was an afterthought i had, and it works very well.

Step 5: Finish!

You should now have a working, on off switchable power supply, (with an on indicator LED), and a clean 3, 5, and 12 volt DC power supply, enjoy modding. (paint if desired, i painted 2 sides and used sandpaper for a brushed aluminum look on the top side.)