Stress Testing ASP.NET Web Apps is all about simulation heavy usage and isolating bottlenecks.
There are some Microsoft Tools available, see below. These tools allow you to create scenarios on your website, increase number of users and view the results.
I'll report back with more information when I actually use these tools. For now this is just a placeholder.
Microsoft Stress Test Tool, ACT, Application Center Test. Available from your Visual Studio Enterprise Edition DVD.
Older Stress Test Tool, downloadable from msdn.
Other useful resources could be Windows Performance Counters. These are Windows information that is available on Processes and Threads running.
You can provide your own applications Performance Counter information be incrementing the Performance counter containers.
The information is Viewable on the Perfmon.exe available on Windows machines.