Mozilla Thunderbird has lots of great features including search, customizable views, IMAP, RSS support and more.
Contacts Sidebar – displays the address books in Thunderbird sidebar. This lets you easily access all your contacts from the main window, quickly edit contact details, drop files on top of a contact to send it as an attachment and more.
GMailUI – is an extension to Thunderbird which add concepts inspired by GMail to Thunderbird, like archiving mail and powerful message searching across all folders. a folder to be your sole “archive” folder, or choose to have an archive folder in each of your multiple accounts.
Quicktext – is an extension for Thunderbird that lets you create templates that can be easily inserted into your own emails. Quicktext is the perfect tool to help you quickly answer routine, repetitive emails. The program can insert different signatures into your emails depending on whom you are sending the email to.