Matt Berseth explains how to create an Outlook like navigation pane in ASP.Net with a combination of an Accordion Ajax Control and the ListView control.
Nice example of how the new Ajax controls in the ASP.NET Ajax Control Toolkit can extend the user experience of regular ASP.Net applications with a minimum of code.

One of the designers on our UI team requested a screen mockup with a page layout that is similar to your typical email client. The page is divided vertically into 2 panes. The left pane contains a 2 level hierarchy of categories and subcategories. As the user selects different subcategories the designer wants the right pane’s content to be updated with the corresponding information. Just like Outlook, the designer wants the subcategories to be displayed within expanding and collapsing panels.

Matt Berseth: Creating an Outlook Navigation Bar using the ListView and Accordion Controls