Email Client

Thundrebird - Thunderbird makes e-mailing safer, faster, and easier than ever before with the industry’s best implementations of features, such as intelligent spam filters, a built-in RSS reader, and quick search.
Eudora - A robust e-mail client whose features include multiple address-book-formatting options and the ability to filter, redirect, and forward mail. It supports QuickTime- and HTML-enriched e-mail, letting you include stylized, formatted text and in-line graphics in your messages.
Opera Mail - Opera Mail is integrated with the Opera browser, and lets you send, receive, sort and search your e-mails quicker and easier than with conventional e-mail programs.
Sylpheed - A surprisingly versatile email client. ‘Surprisingly’ because it has a friendly, easy to use interface to its many useful features.
PopTray - The premier pop-up mail checker for standard POP3 and IMAP mail accounts, including Gmail, and it can be coaxed to work with HTML-only mail such as Hotmail by following the instructions at the PopTray site.