Dane Harnett has released WideMail 0.1.0 for OS X 10.5 Apple Mail users. WideMail splits the Apple Mail main window horizontally, rather than vertically, which really suits the widescreens most Macs come with these days. It also supports Outlook style dual line for each message, which has not been possible until now. WideMail was also easy to install, and works very reliably so far.
In OSX 10.4 Tiger, there was a similar utility called LetterBox, however this has not been updated for Leopard yet. A comment in this MaxOSXHints hint gives a possible way to enable LetterBox in Leopard although I haven't tested this.