cadder :
Luxeon and Cree LED's used in some of the best flashlights can handle voltages in the 6V ballpark, some maybe up to 9V. I haven't kept up with the latest products recently but I don't know if any can go as high as 12V. The more common LED's used in christmas lights, fan lighting, etc. will use less voltage, maybe on the order of 1V each. Typically LED's do have resistors wired in parallel with them, or enough LED's wired in parallel to limite the voltage that each one sees. If you have 5 LED's then you can probably wire them in parallel, but check on the specs for the LED's and you might also need to supply some resistors.
You'd want the resistor wired in series
Also, you'd want the LEDs wired in series if you're not using a resistor - parallel would increase the current draw. You want the voltage across each LED to be just sufficient to get the ideal forward current flowing through it - using Ohm's law and Kirchkoff's laws.
For a 6V LED, you could use the +5V and +12V rails to supply the LED - giving a 7V potential across it. If you do this, it is
really important to use resistors if the LED doesn't have them built in, otherwise you can jeb up the power supply and any 5V components in the PC.
If the LED has a resistor built in, it will only work within a narrow range of voltages. If it doesn't have a resistor built in, then you could run it off almost any voltage provided you used the correct series resistor.
Red LEDs typically have a 1.6V drop across them, so 7 of them
in series (7*1.6V=11.2V) would run nicely off a 12V supply, or three in series across a 5V supply (3*1.6V=4.8V).
You can find the voltage drops for other colours via a quick google search.
You could run a small LED off a mobile phone battery, or off rectified mains power depending on the resistor used - but for mains, this would waste quite a lot of energy through Ohmic heat in the resistor.
-Mark