Page Tracing

What is page tracing?
Tracing allows you to view diagnostic information about a single request for an ASP.NET page.

You can enable page tracing by entering Trace="True" in the page directive at the top of the source view.

Why use page tracing?
Page tracing is a handy way to see values such as session variables at run-time.

Further reading:
How to: Enable Tracing for an ASP.NET Page http://msdn.microsoft.com/en-us/library/94c55d08.aspx
MSDN: Reading ASP.NET trace information http://msdn.microsoft.com/en-us/library/kthye016%28VS.80%29.aspx