Thunderbird is a great email client from the same people who brought you the Firefox browser.
Thunder bird gives you IMAP/POP support, a built-in RSS reader, support for HTML mail,....
powerful quick search, saved search folders, advanced message filtering, message grouping, labels, return receipts, smart address book LDAP address completion, import tools, and the ability to manage multiple e-mail and newsgroup accounts.
Tabbed email
An Archive feature similar to the one in GMail
Lightning fast search
Smart folders
