WMpop is a Window Maker dock app that monitors a local (mbox format) or POP3/APOP mailbox to inform you when mail arrives. It displays the amount of mail in the mailbox (or unread mail if checked in the preference screen), a scrolling text with the email address of the senders (only for local mailbox at the moment) and a status led, indicating mail checking progression and errors. It can be configured and personalized (icons and sound) via a GTK+ GUI.
Pop3eye is a POP3 email notifier. It can be placed in a "dock", or use a regular or undecorated popup window. Regular expressions are used to filter email. It can display up to 20 partial lines of the email body in a tooltip window. It can monitor and display an unlimited number of accounts. You can choose to show messages using regular expressions, or delete messages from the server.