http://mozilla.com/thunderbird/

Thunderbird is an email client available for Windows, Linux and Mac. Built by Mozilla, it provides a clean, easy-to-use system for managing email on the desktop. With advanced spam filtering and search, as well as the ability to add in calendering (which can be synced with Google Calendars), it is a great hub for staying organised. I really love the intelligent feature that reminds you to add an attachment to emails that should have one.