http://www.microsoft.com/downloads/details.aspx?FamilyID=56fc92ee-a71a-4c73-b628-ade629c89499&DisplayLang=en

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.

Features

• HTTP 1.0 and HTTP 1.1 capable
• Supports IPv6
• Multithreaded Support
• Supports generating stress from multiple machines
• Extensible through C plug-in DLLs
• Supports Performance Counter integration
• Measures throughput and response time
• Supports SSL requests
• NTLM Authentication request support
• Easily supports testing thousand of concurrent users
  

Benefits

• Very light weight (low hardware requirements)
• Extensible to handle any aspect of the HTTP request or response
• Allows remote collection of perfmon data and registry
  

Requirements

WCat 6.2 supports the following Operating Systems:

• Windows XP
• Windows Server 2003
• Windows Vista
• Windows Server Code-named "Longhorn"