Letterbox is a plugin for Apple's Mail.app that takes advantage of your widescreen monitor. It rearranges the interface into three vertical columns so the message pane is to the right of the message list, rather than below.
What's new in Letterbox
This version of Letterbox is compatible with Mac OS X 10.6.7 and the build of Mail on new MacBook Pros.
Still incompatible with some Mail plugins that also use Sparkle for auto-updating.
The two-line combined From/Subject column doesn't play well with fonts other than Helvetica and Lucida Grande.
Changing the preview pane position with the menu item or keyboard shortcut does not visibly change the value in the preference pane.