Aaron Harnly wrote a cool plugin to give mail.app vertical panes, a la Outlook. The Letterbox plugin only works in Tiger (OSX 10.4), however a Leopard version is on it’s way.
In the meantime you might want to check out Dan Harnett’s WideMail, which does the same thing, although I’m not sure it’s as nice as I remember Letterbox being. Just something I can’t put my finger on.
Anyway, enjoy.
