A CodePen by Rafał Krupiński.

index.html

<canvas id="c" width="400" height="400"></canvas>

script.js

//settings
var MAX_PARTICLES = 300,
    MAX_PARTICLE_RADIUS = 8,
    CHAR = '@';
    //CHAR = '\u2764';


window.requestAnimFrame = (function(){
  return  window.requestAnimationFrame || 
    window.webkitRequestAnimationFrame || 
    window.mozRequestAnimationFrame    || 
    window.oRequestAnimationFrame      || 
    window.msRequestAnimationFrame     || 
    function( callback ){
      window.setTimeout(callback, 1000 / 60);
    };
})();


var canvas = document.querySelector('#c'),
    ctx = canvas.getContext('2d'),
    cWidth = canvas.width,
    cHeight = canvas.height;

var buffer = document.createElement('canvas'),
    bctx = buffer.getContext('2d');

buffer.width = cWidth;
buffer.height = cHeight;
bctx.font = 'bold 400px/1 Arial, sans-serif';
bctx.fillText(CHAR, 0, 300);



var particles = [],
    particlesCount = 0;

function Particle( x, y ) {
  this.x = x;
  this.y = y;
  this.radius = 0;
  this.lifetime = 0;
  this.color = 'rgba(255, 255, 255, ' + Math.random() + ')';
}

Particle.prototype.update = function() {
  this.lifetime += 0.1;
  this.radius = ~~(Math.abs(Math.sin(this.lifetime) * MAX_PARTICLE_RADIUS));
};


function update() {
  particles.forEach(function( p ) {
    p.update();
  });
  
  if (particlesCount === MAX_PARTICLES) return;
  
  var x = ~~(Math.random() * cWidth), 
      y = ~~(Math.random() * cHeight);
  
  if ( bctx.getImageData( x, y, 1, 1 ).data[3] ) {
    particles.push( new Particle( x, y ) );
    particlesCount += 1;
  }
  
}


function render() {
  ctx.clearRect( 0, 0, cWidth, cHeight );
  
  particles.forEach(function( p ) {
    ctx.save();
    ctx.fillStyle = p.color;
    ctx.beginPath();
    ctx.arc( p.x, p.y, p.radius, 0, Math.PI * 2, false );
    ctx.closePath();
    ctx.fill();
    ctx.restore();
  });
}


(function loop() {
  update();
  render();
  requestAnimFrame( loop );
}());

style.css

body {
  overflow: hidden;
  background: black;
}

#c {
  display: block;
  margin: 0 auto;
}