A CodePen by Tim Holman. Dripping Paint - A little canvas background I made a while back... was the featured background of creativeapplications.net for a small while

index.html

<canvas id='c'></canvas>

script.js

var width = window.innerWidth,
height = window.innerHeight,
c = document.getElementById('c'),
ctx = c.getContext('2d');
c.width = width;
c.height = height;

var paint = [];

var totalPaints = width/50;
var size = 20;

function init(){
    for (var i = 0; i < totalPaints; i++){
        addPaint();
    }
    setInterval( update, 40 );
}

function drawPaint(x,y,size, colour) {
    ctx.beginPath();
    ctx.arc(x, y, size ,0 , Math.PI*2, true);
    ctx.closePath();
	ctx.fillStyle=colour;
	ctx.fill();
}

function update(){
    for (var i = 0; i < paint.length; i++){
        paint[i].y = paint[i].y + paint[i].v;
        if (paint[i].y > height + 60){
            paint.splice(i,1);
            addPaint();
        }
        drawPaint(paint[i].x, paint[i].y, paint[i].s, paint[i].c);
    }
}

function addPaint(){
    //Try 50 times
    var i = 50;
    while(i > 0){
        size = Math.random() * size + 10;
        x = Math.random() * width;

        found = false;

        //Dont Allow drips ontop of each other (Overtaking drops destroy the prettyness)
        for (var j = 0; j < paint.length; j++){
            if ((x + size > paint[j].x) && (x - size < paint[j].x + paint[j].s)){
                found = true;
                break;
            }

            if ((x - size < paint[j].x) && (x + size > paint[j].x - paint[j].s)){
                found = true;
                break;
            }
        }

        if (found == false){
            paint.push({s:size,
                       x:x,
                       y:-60,
                       v:(Math.random() * 2) + 2,
                       c:'#' + (Math.random() * 0x313131 + 0xaaaaaa | 0).toString(16)});
			i--;
            return;
        }
    }
}

init();

style.css

body { background-color: #FFFFFF; margin:0px;}