Google Calendar - Spend less time managing your day & more time enjoying it
Cronofy - Unified Calendar API for developers
Lightning Calendar - Organize your life — it’s about time!
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.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.