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.
Donte The Calendar - Send calendar invites using phone numbers
Planyway - Visual project management for Jira and Trello: manage resources, build project roadmaps, use time tracking.
Corrello - Dashboards for agile teams using Trello
Calendar Screener - Rules to manage your calendar invites.
Wiplo - The easiest way to organize and run your business workflows.