QUOTE(pimpmaul69 @ Apr 9 2006, 10:54 PM)
to drop the voltage correctly on one or even 500 leds in parallel you would use a 100 ohm resistor
First, resistors resist current, not voltage. The more LEDs in a circuit, the more current it requires and the lower the resistor rating you need. If you have 10 LEDs in a parallel circuit, the resistor rating will be lower than if you just have one or two.
If you are wiring one resistor PER LED, then you would use a 100 ohm resistor for each LED, but in a parallel circuit, with one resistor for multiple LEDs, you need to calculate which resistor will provide a safe current, while still giving the LEDs their full brightness. The wattage of the resistor also needs to increase, depending on the load running through it. 1/4 watt is OK for a couple LEDs.