Time.ly - Time.ly is a collaborative calendar solution for websites.
Cronofy - Unified Calendar API for developers
CalendarX - Get a calendar your audience can follow
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.
Ops Calender - Ops Calender is a marketing and editorial calendar to streamline content production, plan better topics, and measure results.