Since upgrading, Snow Leopard users with widescreen displays may have been missing WideMail, the plugin for Mail.app that gives you a 3-column display of your messages. We mentioned it some time ago, and you may recall it being one of the casualties of my Snow Leopard upgrade. Letterbox, an alternative plugin, has been mostly functional in Snow Leopard for a while now, but I've personally been waiting for WideMail. Well, it's back, and available as a public beta at developer Dane Harnett's site.
For users with widescreen displays, the standard Mail display panels -- stacked vertically -- don't really maximize the usage of the window space. WideMail takes advantage of the screen width of most modern displays, and positions the list of mails and the preview horizontally, allowing each to take up the full height of the window while still providing the necessary viewing area. It also provides a new column in the email listing called the "WideMail column," which has two rows and is configurable in the preferences to display any of the necessary information for each email in an optimized width.
As I mentioned, WideMail 2.0 is beta (prerelease 1 right now), but I've been using it for most of the day without any problems. I'm very glad to see it back, and despite trying to keep my Mail plugin list as trim as possible, this is one I consider worth running. If you try it and love it, be sure to donate a few bucks for its continued development!