These are some nice looking, cross browser compliant, ASP.NET based, free controls from Obout.com

Flyout can perform stuff like this:

But almost more interesting is how it can be used in conjunction with traditional controls.  For example, you can wire up a nice looking “Alt Text” effect for images and labels, and you can provide some nice explanation in the flyout for how to properly fill in a control.  The example they give on their site shows a textbox for “Routing Number” and when you click on the textbox it shows this in the flyout:

The alternating content control which is called “Show” (Show examples) can rotate through some content like so:

These are both free controls.  Very nice!