Google Chrome (Get the latest version) allows you to test your websites at various Download/Upload speeds.

This is a fantastic blog to visit if you want to test your websites at varying speeds. You can even create your own throttling profile and choose the download, upload speed and latency.

From Google Web – Optimize Performance Under Varying Network Conditions

Please note Google Chrome is not the only tool/browser  which can do this so fish around if you are not happy with Chrome but I found it pretty cool.