Pop Peeker

From ComputerSmarts.net:

Pop Peeker is a program that runs in your system tray, which allows you to monitor multiple Pop3 mail accounts with out having the hassle of having multiple mail programs open at once. Pop Peeker is designed in such a way to run in your system tray, and check you Pop3 mail accounts at certain intervals of time for new mail. Once new mail is found on the server, you can then preview, reply, or delete the message(s). Pop Peeker doesn't keep any Pop3 connections open to minimize system resources and keep them available for other applications that are open.