So, I recently needed to stress test some code of mine. I downloaded and used the Microsoft Web Application Stress Tool. This is a great and simple tool to help you see how your web sites work under stress. I had one big problem with it - I couldn't get it to work on different ports. Tom Link sent me an email this afternoon pointing out that there is a way, but it's very unintiutive. For each script item, there is a gray box to the left hand side. If you double-click the box, you get a set of options for the script item, including the port.
By the way, another nice free stress tool is JMeter from the Apache people.