Neat. It's like Synergy or PTHiTunesNotifier (now YouControl), but for Mail. That plugin site that the mail.appetizer page links to is very cool as well! http://www.tikouka.net/mailapp/

After using this for an afternoon I find that with a ten-second delay on the floater, I get the gist of each message as it comes in. I only really have to switch to Mail.app when I want to reply!

That's an exaggeration, of course, but I think this is an interesting interface tweak.