"Substitution control use:
By using of Asp.Net 2.0 new Substitution server control, we can dynamically call a method that returns a string in an output-cached web page. Means, the Substitution control will not be cached; even though it’s in output-cached page! Isn’t it interesting?

When we can use this control?
If you have a web page with an output-caching, but still want to display a dynamic string on the output-cached page, then you need this control.
In-other wards, you can output-cache the entire page and then use Substitution controls to specify the parts of the page that are exempt from the caching." (w3coder)

From time to time I still discover about new controls and tricks in ASP.Net 2.0.