Use MailTags integrated tagging interface to add keywords, reminders, and more to your email.
Use Mail Act-On's powerful rule system to make simple keystrokes apply complex actions to your email.
Use Mail Perspectives to turn mailboxes into compact windows and keep critical email close at hand.
Use SigPro to manage and personalize signatures for all your email messages.
Infusionsoft workflow inside Mail
Use Tealeaves to help you read the future of your clients with Infusionsoft, by giving you direct access to their information from within Mail.
The missing manager for OS X Mail plugins
Use Mail Plugin Manager to manage all Mail plugins installed for OS X Mail.