[Plugin: Three Column Screen Layout] Screen options for more columns

Shameless self promotion

Three Column Screen Layout
http://wordpress.org/plugins/three-column-screen-layout
Adds an extra column beside (or below) the main content area on each admin editor screen.

Uses jQuery to late-add an extra container (the others are hard-coded).
CSS makes the post area hidden, and JS later makes it visible, to prevent visible reflow (popping) when the container is first added.

However, any JS failures on the page (caused by other plugins) will halt the code and prevent the form being made visible again. Which may be a good thing, considering your page has just crashed…

Let me know your milage – the hiding/unhiding effect is easily removed.