RadSplitter for ASP.NET AJAX Version Q3 2008 released 11/25/2008 (release history)

You can position the RadSlidingZone at the Left, Right, Top and Bottom ends of the splitter. In any of these cases, the SlideDirection property will give you the ability to slide the panes to the expected direction:

To enable "Left to Right" sliding set the value of the SlideDirection property to "Right".
To enable "Right to Left" sliding set the value of the SlideDirection property to "Left".
To enable "Top to Bottom" sliding set the value of the SlideDirection property to "Bottom".
To enable "Bottom to Top" sliding set the value of the SlideDirection property to "Top".