recently for a website I am working on I wanted to create a horizontal divider capable of resizing two elements on a page using jquery.
Basically:
Content
You may also checkout the UI.Layout jQuery plugin. Here's a demo.
You should just use the jquery resizable interaction : http://jqueryui.com/demos/resizable/
It's easy enough to restrict the dragging areas so you can only resize horizontally (but I think what you actually need is a vertically resizable area)