Whenever - Casual calendar app for iOS
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Avenue - Event messaging app for WWDC attendees
Planyway - Visual project management for Jira and Trello: manage resources, build project roadmaps, use time tracking.
Loxi - Easily add an events calendar to any website.
Corrello - Dashboards for agile teams using Trello