http://www.mozilla-europe.org/en/products/thunderbird/

Thunderbird 3 helps you keep your vital information organised, manage your unruly inbox, and stay informed. By adding different extensions, it scales to the most sophisticated organisational needs while making it easy to find what you need.

Mozilla has bolstered Thunderbird’s acclaimed security and privacy measures to ensure that your communications and identity remain safe. It’s like having your own on-line security guard.

Thunderbird allows you to customise your e-mail to suit your specific needs whether it’s how you search and find messages or listening to music right out of your inbox.

With the lightning extension added it gives you full calendar functions as well.

Useful features include advanced folder views, message tagging (for example “Done”, “To do”, “Later” or your own tags), message browsing (with forward and back buttons), flexible search tools and integration with webmail systems.