One of the biggest irritations of using the built-in Mail application on OS X is the antiquated reading pane that shows up below the message list, especially since most modern email clients let you move the reading pane to the right to take advantage of our widescreen monitors.
There’s a small plugin for Apple’s Mail.app that changes the interface so the reading pane is on the right instead of below. All you have to do is install it, that’s it.
After installation, your reading pane will move over to the right:
If you want to go back to the way it was, run the installer again and choose to uninstall.