For those of you longing for the often-requested three-pane view in Mail.app, Tim Gaden at Hawk Wings has dug up yet another stellar find: an actual working version.
This Ars Technica forum thread begins with the typical "gee, it'd be great if..." post which oh-so-often yields productive results such as the screenshot you see in this post: a hacked version of Mail.app with a three pane view. Fortunately, this hacked version of Mail.app has a different name (MailWidescreen) so you don't have to bite your nails over whether to overwrite your original Mail.app. Still, it would be good practice to back it up anyway for good measure.
Personally, I think the three pane view in email apps is one of those needless 'change for the sake of change' developments that seems to have oozed out of an admittedly old email UI industry. Email subjects can be pretty useful and revealing, while most emails are never long enough to warrant all the space a three pane view gives to the message, and this view seems to enhance both of these complications: subjects are cut short, while white space is wasted on typically short emails. Just note Exhibit A - this post's screenshot.
With my $0.02 out of the way, however, check out the forum thread for details on this hack's development, or simply use the direct link to the .ZIP file right here.