LED Lighting

A 3V LED can be run from a 3V battery without any resistors.
White LEDs will be in this category along with most blue ones.

LEDs have a voltage requirement of 1.5V to 4V and a current of 3 to 1000mA

The resistor required in series with one LED is (from V = I R) :

Voltage - LED Voltage
LED Current mA / 1000