FIXED Thanks to Michael for pointing out the broken link on the download. Now fixed.
Yesterday over at Hawk Wings they posted an entry on how to roll your own custom Mail.app icon, with a link to a .psd template. I personally don’t use Mail.app, rather I’ve been running the nightly build of Thunderbird 3 for quite some time. The nightly is actually called Shredder and is, quite possibly, the worst icon in the world, so when I saw the Hawk Wings post, I jumped at the chance to customize my icon.
I’m not sure anyone else would care to change their icon up, but you can download my alternate Thunderbird icon. Hawk Wings has a link in that post on how to change icons in Mac OS X for those new to Mac.