Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Roundcube - Web-based IMAP email client
Front - multi-channel inbox service for collaborative teams
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.