Apple’s Mail.app version 2.0 is a drastic change from the previous version. I’m not totally sold on the new look. Here are some free applications that will hack the resources of mail.app so that it looks different.
Mail Fixer, looks like it’ll work with themes so users can create their own theme and share them with other users of the app.
if you come across other hacks for mail.app then post in the comments.