EzPop is a very nice, compact POP3 e-mail notification tool. In addition to checking for new e-mail and displaying headers it also provides the ability to read and delete e-mails currently on the server. The interface is very simple and it supports custom skins. The developer website does not appear to be actively maintained but you can find many download sites that have it. EzPop is free. The screenshot above shows EzPop using the GT3 Reloaded skin, which I downloaded from a skins site.
I have not had any problems with the program so far and the ability to skin the program helps it blend in well with running widgets.