Google Contacts - Google Contacts is Google's contact management tool that is available in its free email service Gmail, as a standalone service, and as a part of Google's business-oriented suite of web apps Google Apps.

Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!

Google Calendar - Spend less time managing your day & more time enjoying it

Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.

Apple Contacts App - Access and edit your contacts from personal, business, and other accounts with the Contacts app.

Mailspring - Mailspring is a mail client for Mac, Windows and Linux.