I find the Mail.app plugin Widemail extremely useful. At my day job I use Outlook and the widescreen view is very efficient for space management. Prior to upgrading to Leopard, I used the application Letterbox written about here; however in my recent search for a Mail-enhancing plugin I wanted something a little more robust. Enter Widemail. It not only provides the awesome widescreen view from Letterbox, but it provides a means for creating a two-line message row similar to that found in Outlook for Windows and Entourage on the Mac.
Since we last wrote about Widemail the developer has added a preference pane within Mail.app making it easier to tweak settings. Additionally, the Widemail custom column can now have items that are left-aligned and right-aligned within the same row. Everyone's favorite automatic self-update framework, Sparkle, has been added as well.
Widemail makes Mail.app exceptionally more functional for my day-to-day use and is a free download (as in, feel free to make a donation).