Hidden features of ASP.NET

On the StackOverflow forum, you'll find questions like "Hidden features of" technology X to be popular with a lot of up-votes. Such questions are sometimes closed as they are too subjective & do not fit into the expected Q&A format of the site. They however contain insightful answers.

Here is a list of "hidden" gems in ASP.NET that I've discovered (work in progress) and wanted to highlight them as the don't easily show up in the top results while googling -

• Tag property for server controls - Just like the rel HTML attribute can be added to an element to store extra  info when it is required in a client-side programming scenario, the Tag property can be used with server controls to store some extra info that may not fit into regular properties.
• RepeatLayout Property of  CheckBoxList server control - this property used to allow 2 possible values "Table" and "Flow" till ASP.NET 3.5, but from version 4 onwards it supports OrderedList and UnorderedList as well.  If you need to style a group of checkboxes generated through the  CheckBoxList server control, the default "Table" layout may be restrictive.  "Flow" and the other new options provide more flexibility in modifying the layout through CSS.

Related:
StackOverflow: Hidden Features of ASP.NET