Whenever - Casual calendar app for iOS
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Avenue - Event messaging app for WWDC attendees
Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
Loxi - Easily add an events calendar to any website.
pretix - pretix helps you to sell tickets for your event in an easy way. It supports multi-lingual events and provides a wide range of features