Web Capacity Analysis Tool (WCAT)

Web Capacity Analysis Tool (WCAT) is a lightweight HTTP load generation tool primarily designed to measure the performance of a web server within a controlled environment.  WCAT can simulate thousands of concurrent users making requests to a single web site or multiple web sites.  The WCAT engine uses a simple script to define the set of HTTP requests to be played back to the web server. Extensibility is provided through plug-in DLLs and a standard, simple API.

Links:

http://www.iis.net/community/default.aspx?tabid=34&i=1466&g=6