If you are a web developer, I recommend that you drop everything right now and go install Nikhil Kothari's Web Development Helper.  It is a fabulous tool for writing and debugging ASP.NET applications.  It helps with in the following areas:

• Tracing
• ViewState
• Caching (ASP.NET and HTTP)
• Restarting AppDomains
• Logging HTTP(S) Traffic (Including XMLHttpRequest)
• DOM
• JavaScript

...and I just realized that Scott Guthrie has also blogged about the Web Development Helper.  For more info, see his post.

For more developer resources (web and otherwise), see my Developers Tools List.