MailCore is an Objective-C framework for iOS and Mac OS X built to ease the pain of supporting the IMAP and SMTP protocols. With MailCore you can download e-mail, create folders, delete folders, rename folders, send e-mail and etc.
Rocketbox provides wickedly fast e-mail search for Mail.app on Mac OS X. Has a person search feature that makes seeing correspondence with a particular person a cinch. Since Mac OS X Lion this app has been retired.