Mozilla Thunderbird is a redesign of the integrated Mozilla App-Suite mail component. Below is a breif list of some of the changesin 0.2:

-Imporved Footprint and Performance
-Improved Default Theme
-Improved Spell Checker
-Redesigned Options Dialog
-Improved customization
-Improved Stability

The release notes can be found here: http://www.mozilla.org/projects/thun...ase-notes.html
The Project's Website: http://www.mozilla.org/projects/thunderbird/index.html

At this time Thunderbird is still concidered pre-Alpha and should be treated as such. But I have been using Thunderbird as my default e-mail client on both Windows and Linux for a while without any problems.

Windows: http://ftp.mozilla.org/pub/thunderbi...-0.2-win32.zip
Linux with GTK2 and XFT: http://ftp.mozilla.org/pub/thunderbi...k2-gnu.tar.bz2
Mac OS X: http://ftp.mozilla.org/pub/thunderbi...-macosx.dmg.gz

the 0.2 Source Tarball is also available here