detect user bandwidth!

detect.js

var imageAddr = "http://www.tranquilmusic.ca/images/cats/Cat2.JPG" + "?n=" + Math.random();
var startTime, endTime;
var downloadSize = 5616998;
var download = new Image();
download.onload = function () {
    endTime = (new Date()).getTime();
    showResults();
}
startTime = (new Date()).getTime();
download.src = imageAddr;

function showResults() {
    var duration = (endTime - startTime) / 1000; //Math.round()
    var bitsLoaded = downloadSize * 8;
    var speedBps = (bitsLoaded / duration).toFixed(2);
    var speedKbps = (speedBps / 1024).toFixed(2);
    var speedMbps = (speedKbps / 1024).toFixed(2);
    alert("Your connection speed is: \n" + 
           speedBps + " bps\n"   + 
           speedKbps + " kbps\n" + 
           speedMbps + " Mbps\n" );
}

It's possible to some extent but won't be really accurate, the idea is load image with a known file size then in its onload event measure how much time passed until that event was triggered, and divide this time in the image file size.
Quick comparison with "real" speed test service showed small difference of 0.12 Mbps when using big picture.

http://stackoverflow.com/questions/5529718/how-to-detect-internet-speed-in-javascript