Phatso is a widescreen-layout plug-in for Apple Mail 3.0 (Leopard). All it does is makes your Mail look like this:

Unzip and copy Phatso.mailbundle to ~/Library/Mail/Bundles (you may need to create this folder). Then, run the following commands in Terminal:
defaults write com.apple.mail EnableBundles -bool YES defaults write com.apple.mail BundleCompatibilityVersion -int 3
If you drag the divider between the message list and the preview pane to the right edge of the window, the preview pane will be hidden. To get it back, drag the right edge (admittedly, it's a bit tricky to get the pointer right onto the edge) of the window back into the message list.
Vernon Tang