ROBOT.LK

Here is a nice site that calculates the resistor required to connect an LED to a power source.
The simple calculation is R = (V_S - V_L) / I
Resister wattage: (V_S - V_L) x I

My Values were:
Source voltage: 230
diode forward voltage: 2*
diode forward current (mA): 20*
*See 5mm LED typical technical data

This was the result which is about right. Have a look: http://led.linear1.org/1led.wiz

Practically I've used 100k, 1/2W resistor with a LED. Current flow is too small, but it's suitable when you use LED for indication.

I have used this in lot of places as a indicator...As my experience 120k/0.25W is the best one.
Rksk's resistor is 0.5W ... spend more money ...
And,newer use 10k/0.25W .... because,it is work.but,some days resister became leak...LED is lost

**This all are valid for RED stranded and bright(white-red) LEDs.