Aaron Harnly’s Letterbox addon for Apple’s Mail.app:

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.

This makes me so, so happy. I had this working in previous versions of OSX and it was one of the first things I tried after upgrading to 10.4.. Seeing more messages is a big productivity boost for me because things that need attention don’t get pushed out of sight.
I’m glad someone finally got it working, thank you Aaron. I hope he does eventually post the source code, I’m curious to see how he did it (and if it changed from the Ars posting).

update URL updated.