MailCountX adds an icon to the OS X system status bar (right next to the clock, AirPort icon, etc) that indicates how many unread messages are in your Mail.app inboxes. Features include: display unread mail count in mailboxes named "INBOX" optionally display unread mail count in all mailboxes optionally remind user of unread mail at user-specified interval by playing a sound alert option can be configured to play a standard system sound or a user-defined audio file | 
