Google Calendar - Spend less time managing your day & more time enjoying it
HEY - Email at its best, new from Basecamp.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
InstAddr - Disposable email address application.
iCal - In Calendar on your Mac, subscribe to calendars published or shared by others.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.