From Manco Software:
The Site Test Center application is designed to automate the process of testing of Web Sites, Web Applications, and Web Services. It allows one to perform stress tests and tests the functionality of server side parts of applications. Using flexible and powerful scripting models you can create testing scripts that will simulate a regular user session or test particular server parts of you web application. The distributed testing model allows you to create testing environments that will suit your needs. You can use a single PC to run some kind of functionality test during development and QA stages of the project, as well as multi-PCs environments to perform stress and load tests. Script execution can be started immediately or scheduled to start at any given time, and can be stopped after a given number of runs, or at a given time, or after a given number of the errors.