Plan - Plan is a better way to organize.
Cronofy - Unified Calendar API for developers
Streaks - The to-do list that helps you form good habits.
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.
Taskful - Deadlines, meet your match.
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.