Thunderbird is a free, open source, cross-platform e-mail, news and instant messaging client. The project strategy is modeled after Firefox.
It supports POP (storing mail locally on your hard drive so that it can be accessed without an internet connection), and IMAP. It features excellent mail filter capabilities and management.
Thunderbird has strong support for using multiple accounts and identities, including automated signature features.
Chat functionality is built in, so you can enjoy real-time conversation with your contacts, right from your favorite messaging application, with multiple supported networks. Thunderbird makes it easy to search through both past conversations and received emails.
Thunderbird can also function as an RSS/Atom feed reader, and has a large selection of add-ons that extend it with calendar support, PGP support, integration with online services and much more.