Productivity enhancements for OS X mail

I've always liked Mail because it's fast and reliable, but one of the jobs I've never got around to is pimping the interface to make it more usable - until now thanks to being inspired by this screenshot on Flickr.

The components used to create this set up are:

Mailtags - tagging messages and making them time sensitive
Letterbox - provides a three column layout

and lastly the use of smart mailboxes to give fast access to useful messages.