RGB to Luminosity (140byt.es)

annotated.js

function(
    r,          // red, as a number from 0 to 255
    g,          // green, as a number from 0 to 255
    b,          // blue, as a number from 0 to 255
    p           // as goog a place as any to declare p 
){
    p = Math.pow;                // alias for "Math.pow"
    return                       // return a luminosity value madeup of
    0.2126 * p(r/255, 2.2) +     // the red value plus
    0.7152 * p(g/255, 2.2) +     // the green value plus
    0.0722 * p(b/255, 2.2)       // the blue value
}

index.js

function(r,g,b,p){p=Math.pow;return 0.2126*p(r/255,2.2)+0.7152*p(g/255,2.2)+0.0722*p(b/255,2.2)}

LICENSE.txt

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                    Version 2, December 2004

 Copyright (C) 2011 YOUR_NAME_HERE <YOUR_URL_HERE>

 Everyone is permitted to copy and distribute verbatim or modified
 copies of this license document, and changing it is allowed as long
 as the name is changed.

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. You just DO WHAT THE FUCK YOU WANT TO.

package.json

{
  "name": "RGBtoLum",

  "description": "Calculate the luminosity of a colour based on its RGB values",

  "keywords": [
    "RGB",
    "colour",
    "color",
    "Luminosity",
    "Calculation"
  ]
}

looks cool! by omitting the leading 0 on floats .34 you can save some more bytes! Plus that makes it possible to save another byte if you leave out the space after return!