You've written your web application and tested it to ensure it all works but how do you know it will be able to take the strain when you launch it on an eager world?
Load testing. That's the key. There are many web application load testing tools around. Some are free and some cost an awful log of money. There have been and still are a number of tools available from Microsoft with varying richness of feature sets.
Visual Studio Team Suite and Visual Studio Team Edition for Testers both include a completely new web load testing tool.
If you are no sure which versions of Visual Studio 2005 include what read the product comparison.
Changing a Web Vanguard – Windows Live HotmailPerformance Testing Guidance for Web ApplicationsVisual Studio 2005 Team Test Edition: Testing Demos
Main blogs for VSTS Load Test
Other test tools
WCat 6.3 - lightweight tools for applying load to web sites. Available in x86 and x64 versions.WatiN - framework for automating the browser UIVSTS Scriptable web test parameters plugin