Hidden Features of ASP.NET [closed]

This question exists because it has
historical significance, but it is not
considered a good, on-topic question
for this site, so please do not use it
as evidence that you can ask similar
questions here.

More info: https://stackoverflow.com/faq

There are always features that would be useful in fringe scenarios, but for that very reason most people don’t know them. I am asking for features that are not typically taught by the text books.

What are the ones that you know?

While testing, you can have emails sent to a folder on your computer instead of an SMTP server. Put this in your web.config:

<system.net>
    <mailSettings>
        <smtp deliveryMethod="SpecifiedPickupDirectory">
            <specifiedPickupDirectory pickupDirectoryLocation="c:\Temp\" />
        </smtp>
    </mailSettings>
</system.net>