Here is a FREE web stress tool from Microsoft which is designed to simulate multiple browsers requesting pages from a web site. Yhis tool can be used to gather performance and stability information about the web application under test. This tool simulates a large number of requests with a relatively small number of client machines. The goal is to create an environment that is as close to production as possible so that you can find and eliminate problems in the web application prior to deployment.
URL: http://www.microsoft.com/downloads/details.aspx?FamilyID=e2c0585a-062a-439e-a67d-75a89aa36495&displaylang=en
An article on this can be accssed at: http://www.west-wind.com/presentations/webstress/webstress.htm
-Devankur