A simple script to make falling snow on any page

snowflakes

(function() {
  var snowflakes = [],
      moveAngle = 0,
      animationInterval;

  /**
   * Generates a random number between the min and max (inclusive).
   * @method getRandomNumber
   * @param {Number} min
   * @param {Number} max
   * @return {Number}
   */
  function getRandomNumber(min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
  }

  /**
   * Creates a new snowflake div and returns it.
   * @method createSnowflake
   * @return {HTMLElement}
   */
  function createSnowflake() {
    var el = document.createElement('div'),
        style = el.style;

    style.borderRadius = '100%';
    style.border = getRandomNumber(1, 4) + 'px solid white';
    style.position = 'fixed';
    style.zIndex = '999999';
    style.boxShadow = '0 0 2px rgba(255,255,255,0.8)';
    style.top = getRandomNumber(0, window.innerHeight) + 'px';
    style.left = getRandomNumber(0, window.innerWidth) + 'px';

    return el;
  }

  /**
   * Calls the moveSnowflake method for each of the snowflakes in the cache.
   * @method moveSnowflakes
   * @return {Void}
   */
  function moveSnowflakes() {
    var l = snowflakes.length,
        i;

    moveAngle += 0.01;

    for (i=0; i<l; i++) {
      moveSnowflake(snowflakes[i]);
    }
  }

  /**
   * Moves an individual snowflake element using some simple math.
   * @method moveSnowflake
   * @param {HTMLElement} el
   * @return {Void}
   */
  function moveSnowflake(el) {
    var style = el.style,
        height = window.innerHeight,
        radius,
        top;

    radius = parseInt(style.border, 10);

    top = parseInt(style.top, 10);
    top += Math.cos(moveAngle) + 1 + radius/2;

    if (top > height) {
      resetSnowflake(el);
    } else {
      style.top = top + 'px';
    }
  }

  /**
   * Puts the snowflake back at the top in a random horizontal start position.
   * @method resetSnowflake
   * @param {HTMLElement} el
   * @return {Void}
   */
  function resetSnowflake(el) {
    var style = el.style;

    style.top = '0px';
    style.left = getRandomNumber(0, window.innerWidth) + 'px';
  }

  /**
   * The kick-off method. Asks how many snowflakes to make and then makes them!
   * @method setup
   * @return {Void}
   */
  function setup() {
    var number = prompt('How many snowflakes would you like?'),
        particle,
        i;

    // Setup snow particles
    for (i=0; i<number; i++) {
      particle = snowflakes[i] = createSnowflake();
      document.body.appendChild(particle);
    }

    // Set animation intervals
    animationInterval = setInterval(moveSnowflakes, 33);
  }

  setup();
}());

I changed again the script and I released a script that generates Christmas Balls, hope you find it nice! Thanks!

/*
* https://gist.github.com/trentmwillis/2199d6d191000b8d8f40#gistcomment-2749653
* Modified javascript to make rain of Christmas balls... the effect is amazing!
*/

(function() {
  var christmasballs = [], moveAngle = 0, animationInterval;
  var browserWidth = document.body.clientWidth;
  var browserHeight = window.innerHeight;
  /**
   * Generates a random number between the min and max (inclusive).
   * @method getRandomNumber
   * @param {Number} min
   * @param {Number} max
   * @return {Number}
   */
  function getRandomNumber(min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
  }
  /**
   * Generates a random color.
   * @method getRandomColor
   * @return {Color}
   */
  function getRandomColor() {
    var letters = '0123456789ABCDEF';
    var color = '#';
    for (i=0; i<6; i++) {
      color += letters[Math.floor(Math.random() * 16)];
    }
    return color;
  }
  /**
   * Creates a new christmasball div and returns it.
   * @method createChristmasball
   * @return {HTMLElement}
   */
  function createChristmasball() {
    var el = document.createElement('div'), style = el.style;
    style.borderRadius = '100%';
    style.border = getRandomNumber(4, 8) + 'px solid ' + getRandomColor();
    style.position = 'fixed';
    style.zIndex = '999999';
    style.opacity = '0.9';
    style.filter = 'alpha(opacity=90)';
    style.top = getRandomNumber(0, browserHeight) + 'px';
    style.left = getRandomNumber(0, browserWidth) + 'px';
    return el;
  }
  /**
   * Calls the moveChristmasball method for each of the christmasballs in the cache.
   * @method moveChristmasballs
   * @return {Void}
   */
  function moveChristmasballs() {
    var l = christmasballs.length, i;
    moveAngle += 0.01;
    for (i=0; i<l; i++) {
      moveChristmasball(christmasballs[i]);
    }
  }
  /**
   * Moves an individual christmasball element using some simple math.
   * @method moveChristmasball
   * @param {HTMLElement} el
   * @return {Void}
   */
  function moveChristmasball(el) {
    var style = el.style, height = browserHeight, radius, top;
    radius = parseInt(style.border, 10);
    top = parseInt(style.top, 10);
    top += Math.cos(moveAngle) + 1 + radius/2;
    if (top > height) {
      resetChristmasball(el);
    } else {
      style.top = top + 'px';
    }
  }
  /**
   * Puts the christmasball back at the top in a random horizontal start position.
   * @method resetChristmasball
   * @param {HTMLElement} el
   * @return {Void}
   */
  function resetChristmasball(el) {
    var style = el.style;
    style.top = '0px';
    style.left = getRandomNumber(0, browserWidth) + 'px';
  }
  /**
   * The kick-off method. Asks how many christmasballs to make and then makes them!
   * @method setup
   * @return {Void}
   */
  function setup() {
    var number = prompt('How many Christmas balls would you like?'), particle, i;
    // var number = 150, particle, i;
    // Setup snow particles
    for (i=0; i<number; i++) {
      particle = christmasballs[i] = createChristmasball();
      document.body.appendChild(particle);
    }
    // Set animation intervals
    animationInterval = setInterval(moveChristmasballs, 33);
  }
  setup();
}());