Cronofy - Unified Calendar API for developers
fruux - Free service that takes care of your contacts, calendars and more so you don't have to.
ApPHP Calendar - ApPHP Calendar is a PHP calendar script that may be easily integrated and used with various PHP projects, such as schedulers, event, etc.
FullContact - Put the power of full contact information into your web or mobile apps.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
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.