DRDUL  I just switched to a Mac from Windows, and from Outlook to Apple Mail (via Outlook2Mac). Being the paranoid tinfoil-hat-wearing person that I am, I keep an archive of three years worth of mail. VacuumMail saved my bacon, as Apple Mail was choking on the huge number of message I imported and then started moving around. I use it regularly (it also runs automatically) to keep Mail "fit and trim." Highly recommended!  
(Version 0.9.6)

TUISHIMI  Works as advertised and my mail runs faster and smoother. Not only that, since I ran this I no longer suffer from the "can't connect to server" problem with mail!!! I don't know if it is coincidence or something else, but I don't care. I am so happy I don't have to go offline/online all the time now.  
(Version 0.9.5)

APPLEDOGX  WOW!!! Incredible and surprising. Mail was a bit sluggish to start on my Mac for many months. After running this quick utility, now it starts up in two seconds maximum!!!!

This product is an example of simplicity being elegance. Kudos to the author.  
(Version 0.9)

EAN  Fantastic. Does its job quickly and Mail is now noticeably faster launching and navigating around mailboxes. A progress bar and prettier dialogs would be nice, but the 'architectural' design, complete with launchd support, is superb.  
(Version 0.9)