Ease-E-Check
"Ease-E-Check is an easy to use e-mail checking program that sits as an icon in your system tray. It can check an unlimited number of pop3 e-mail accounts and supports skins and sounds. (...) There are three main sections of Ease-E-Check. The System Tray Icon which notifies you of new mail and allows you to quickly interact with Ease-E-Check. The QuickBar is a customizable window that allows you to easily perform most of Ease-E-Checks main functions right on your desktop. (...) Ease-E-Check supports two types of skins, System Tray Icon skins and QuickBar Skins."
Status: active.
EzPop
"Developed by Thibaud Djian of Hoverdesk fame, EzPop is a free skinnable email/news checker. Its main feature is to check your mail accounts every 1 to 99 minutes. When you have mail, EzPop will warn you by a mailbox image, a sound or a pop-up."
Status: not under development anymore.
neoMail
"neoMail is a POP3 email checker. neoMail will check all your email accounts without any intervention. And when new mail is found, neoMail will notify you."
Status: dead. By former Hydra software; link points to our archive.
SkinPOP3
A skinnable mail checker, displaying number of mails on multiple accounts. Other features are sound, alphablending, preview of mail header and executing a specified mail client.
Status: active.