Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Meekan - Connecting the world's calendars
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.
Sidequest for Slack - Create and track personal or support team tasks, 100% inside Slack.