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.

If you don't have a version that include the test tool but want to try it out then, at the time of writing, you can obtain a 180-day trial version.

Once you have it you may be looking for some guidance on how best to use it. Known by its code name "Ocracoke" before it was released, the web testing features of Visual Studio 2005 are easy to use but also quite extensive and therefore not something you can learn fully in a couple of hours.  The following are a set of links that I hope you find useful

Changing a Web Vanguard – Windows Live Hotmail
Performance Testing Guidance for Web Applications

Visual Studio 2005 Team Test Edition: Testing Demos

Web Test Authoring and Debugging Techniques

Working with Web Tests (online documentation)
Visual Studio Team System - Testers (forum)
Web and Load Test FAQs
So you want to replay an IIS web server log?
Distributed Functional Testing with Visual Studio 2005 Team Test Load Agent

MSDN Webcast: Load and Web Testing with Microsoft Visual Studio 2005 Team System (Level 200)

 

Main blogs for VSTS Load Test

http://blogs.msdn.com/vstsloadtestblog
http://blogs.msdn.com/joshch/
http://blogs.msdn.com/edglas/
http://blogs.msdn.com/slumley/

 

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 UI
VSTS Scriptable web test parameters plugin