Counterpart to Mozilla Firefox, Thunderbird is a stand-alone e-mail program similar to the one in plain vanilla Mozilla.
While highly extendable, yet built with ease of use in mind, Thunderbird's real selling point is the spam blocking feature. It also comes with built-in spell checking and support for S/MIME and other secure e-mail. (Though I personally use the EnigMail extension, since I prefer PGP.) Like Firefox, Thunderbird supports the use of themes, and is cross-platform, with releases available for Linux, Mac OS, and Microsoft Windows.
The only fault of Thunderbird (and Mozilla Mail) is the address book, which is buggy and not very user-friendly. This might change in future versions, but for now I still use the Windows address book.
It is available for download at <http://www.mozilla.org/products/thunderbird/>, and you can find extensions, themes, and help at <https://addons.mozilla.org/thunderbird/>.