I use Thunderbird as a mail client at the office for my personal email—mainly because it’s free; I hate all email clients—and I just noticed that v1.5 shows a unique icon: if you have both forwarded and replied to an individual email, the icon has both a left-pointing green arrow for a reply and a right-pointing purple arrow for a forward. Every other mail client I’ve used—and I’ve used Outlook, Netscape, Eudora, Mulberry, Thunderbird, and Mail.app—overwrites the message icon with whatever the last action was, which is not good.

Awesome: knowledgeable metadata in one simple icon. Kudos.