I've seen "The Bat!" pop up frequently in computer magazine CDs as well, not sure how widely it was used though.

The Bat! was absolutely the best email client. ever. way ahead of eudora. it was a massive step back when i switched to my first macbook in 2006 (the black one!) and started to use Thunderbird. That said Thunderbird is fantastic now and great to see it get native Exchange support!