Here is a thread on http://forums.iis.net that discusses Stress testing tools. There is a wide variety tools available. I personally use a powershell script to create a single log file, then load test with Web Application Stress tool (retired by Microsoft). For my personal needs this has been sufficient. I thought I would pass along as an FYI.
If you have tools that you’ve found successful, feel free to add to the comments section
PS – And a great tool to help analyze the data is using PAL (performance analysis of logs)
Windows Server MVP – IIS
Enterprise Log Management solution
Install, Configure, Forget