Martin Krüger wanted to create something useful with the Silverlight GridSplitter control and ended up with this GridSplitter Picture Viewer. You can view the sample and download the source code at the Expression Gallery.
Source: Microsoft Expression Gallery
A simple solution for a picture viewer where the aperture is implemented using a GridSplitter control.
This is no UserControl or CustomControl. But the technique using a GridSplitter could be used for a User/CustomControl.