Poptray is a great e-mail checker that is free and works great. The application will sit on your tray and constantly scan your e-mail accounts searching for new mail. Poptray will let you know when new mail comes in so that you can check it on your e-mail client or on the built-in previewer. The built-in viewer is not too complete, but it is great for quickly checking incoming e-mails to see who sent them and what is on them. You can open your preferred application from that window. There are a bunch of features that make this application amazing. I personally like that it can display the number of unread messages directly on the icon, a la Mac. Also, you can have as many accounts as you want on the client. The number of e-mails that are unread will keep on rotating. You can also configure personalized notifications and hot-keys for easy access. There is extensive support for plug-ins and add-ons. Since the application is open-source, you can download the code and compile it yourself, making any changes you want along the way.
- Very much customizable. Support for unlimited accounts