Create a second Mail window to watch a specific folder

For some time, Mail has featured an awkwardly named File > New Viewer Window command (Command-Option-N). This opens a second, fully functional Mail window, complete with the ability to hide the sidebar (View > Hide Mailboxes, or Command-Shift-M) to cut down on visual clutter.

This is useful if you want to keep an eye on both your Inbox and, say, a high-volume Smart Folder, a mailbox that messages are dumped into by a filter, or a folder that you use as a to-do list.

The advantage of a second Mail viewer window is that you don’t have to frequently mouse back and forth between folders. You can simply keep both windows open side-by-side (perhaps on their own Space in Leopard or Snow Leopard), or easily swap between them with Mac OS X’s Command-` windowing shortcut.