Teamup Calendar - Cloud-based calendaring using capability URL, application developed in Zurich, Switzerland.
Gone - An ephemeral to-do list
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
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.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.