That's the subject of our new poll. I last asked this question in May of 2004, and at that time, Mail had a whopping 70% share, followed by Entourage and Eudora.
It will be interesting to see if Gmail and/or Thunderbird have made up any ground since then...
-rob.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060609140157470