Check Mail (POP) is a very lightweight and easy to use utility for checking for new mail in a POP account. It is highly customizable allowing you to be notified in a number of ways.

Check Mail (POP) Features
Compatible with any account that can be accessed via POP
Don't like my icons? Use your own!
Play a custom new mail sound of your choice, either wav or mp3 format
Optional pop-up alert window with a time delay of your choice
SSL compatible so can be used to check on your new GMail account
Small program that uses relatively few resources (compared to a full mail client)
The option to launch a webpage and/or an installed full mail client
Discreet (Tray icon) existence