Careful planning and development are necessary for any Web application development project. However, in order to make a truly scalable Web application, it is important that you rigorously and regularly test it for scalability problems. You should remember that almost all Web applications encounter some unforeseen scalability problems during development. The only way these problems can be found and solved is through careful, thorough testing.
In general, the most useful scalability and performance testing stresses the application in a way that closely parallels the actual stresses the application would undergo in a "real-world" setting.
Microsoft® Web Application Stress Tool is a Web stress tool that is designed to realistically simulate multiple browsers requesting pages from a Web application. It can be used to generate customizable loads on various Internet services, and offers a rich set of features, that are desirable for anyone interested in gathering performance data for a particular Web site.
The Web Application Stress Tool is available on the Windows 2000 Resource Kit Companion CD.