Today I learned some interesting things about power supplies and basic electrical precepts. Like most I already knew that the yellow wires are 12 volts and the red wires are 5 volts, and for your average case mods that's all you really need to know about. Keep in mind the wire colors I talk about are in a normal industry standard power supply. I've come across a lot of Dell power supplies that use orange as the 5V line....weird.
When I purchased a set of LED feet for a case mod I didn't give it a second thought when I started cutting and rewiring to incorporate the feet into my rig. The feet plugged into the 5V line; and, to my surprise, there were small resistors in line with the wire I had cut that reduced the 5V to 3.3V (I thought I was cool enough to have 5V LED's). So those LED's didn't last long. In fact they lasted all of about a minute before they burnt out, but for that minute they were so cool. The orange wires that go to the rarely used ATX auxiliary connector (looks a lot like the old AT style power connections) are 3.3 volts. Now not only do I not have an excuse when I burn out LED's I finally have a use for that cable.
I found some more information at this link.