Mozilla Thunderbird is a free, open source, cross-platform email, news, and chat client developed by the Mozilla Foundation. Thunderbird is an email, newsgroup, news feed, and chat (XMPP, IRC, Twitter) client.
OS: Windows, Mac, Linux, BSD.
Claws Mail is a free and open source, GTK+-based email and news client. It offers easy configuration and an abundance of features. It is included with Gpg4win, an encryption suite for Windows.
OS: Windows, Mac, Linux, BSD, Solaris, Unix.