When people post a comment on this site and list a URL, I often visit out of curiosity. OK, I'm nosy--I can't help it. Sometimes it takes a while to get around to looking, now that my blogroll is reaching unmanagable proportions. Anyway, that was how I came upon this absolute gem of an Applescript, on Pete Boardman's weblog.
I was slightly missing the feature in Apple Mail which displays the number of unread mails in the Dock icon, but this is even better. It uses GeekTool to display the name of each mailbox with unread mail, and the number of unread mails in each box. I have it set up to display in the bottom right corner of my desktop, so that it's always visible. It's fantastic--if I don't happen to be on VirtualDesktop 3 (which is where Mailsmith lives), and Mailsmith goes 'ting' to tell me new mail has arrived, I can see if it's likely to be worth switching desktops for, or if it's from the work mail-list, which is rarely worth switching back for.