Hover Menu Extender
The hover menu extender is a great extender in the AJAX Control Toolkit. Essentially, it is an extender that will appear whenever you hover over an HTML element. I use it to show helpful tooltips that exist in a panel, so when you mouseover a label control (requires a server control to connect to), a helpful message will appear in my form explaining what they need to provide in a certain field. To do this, you have to create a panel, with the tooltip. I use a custom skin to change the color of the panel to whatever style I want. I add a style of "display:none" just in case to ensure it is hidden, but I've noticed when I forget that, the toolkit seems to do it for me.
For the hover menu extender, you set the TargetControlID to the label you want this panel to appear over, and the PopupControlID to the panel ID itself. Remember all of these must be server controls, and must have an ID that you can link to. You can specify a PopupPosition to display to the left, right, over top, above, or below the label control (through an enumeration). You can even specify a delay as well.
I tried an animation, and was told that would come in a future release. I haven't tried the RTM to see if animations are supported yet.