RelatedMail is a nice little add on to your Apple Mail. This plugin shows up next to a selected message and shows all related emails by date, subject or sender.
RelatedMail is a little difficult to install, as you'll not only need to copy the folder into your library, but also enter a few Terminal commands.
Once installed, open up Mail, select View and press 'Show Related Messages'.
I did find RelatedMail lacking in explanations. For most things, you'll find you need to go to the developer page to get more information.
I would have also liked to be able to reorder related messages myself by choosing the field. RelatedMail seems to have a mind of its own though, both in the way it indexes and shows related messages.