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, IMAP (including uploads, albeit a date issue), NNTP and RSS and chat support for IRC, Twitter, XMPP and Google Talk. Having OAuth support, it can be used with accounts that require it instead of a password.
Mozilla Thunderbird, Portable Edition is a special build licensed by Mozilla. Alternatively, X-Thunderbird is also available.