Mozilla Thunderbird is a skinnable, feature-rich email, calendar, task and instant messaging client with support for plugins and many protocols. It comes with advanced search tools, message filtering, Bayesian spam detector, built-in spell checker, mailbox compression and many more. Program features can be extended with add-ons, including a specialized version of email encryption tool GPG.
Email protocols include SMTP, POP3, IMAP4, NNTP and RSS and chat support for IRC, Twitter, XMPP and Google Talk.
Mozilla Thunderbird, Portable Edition is a special build licensed by Mozilla. Alternatively, X-Thunderbird is also available.