The web part that I'm presenting today is more of a building block, it doesn't do much on its own. It makes use of the JQuery Corners Plugin.
To set the corner types you can modify the Corner Type setting:
The above example will cause the following JavaScript to be output:
jQuery('cornerDiv').corner('tr 25px');
This web part makes use of the JQuery Script Manager that I've posted about previously.
As usual the source can be found here.
A screencast of the control in action can also be found here.